Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: zippy! on April 25, 2017, 08:10:20 PM
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Hi all, firstly sorry to drop in and be a pain! I hope the moderators allow my post.
I have had help before with missing bits and thanks to you guys being a great community I was helped out massively.
I am running some CB750 K7 carbs on a Hillman Imp. Yes you read that right! I bought them off an imp trader and thought I would give them a go, and prevailed, despite many a time I could have given up.
The car has only done a few hundred miles since restoration and bar needing a proper tune, it was running fairly well, albeit a bit rich. Pain to start due to the chokes being removed but once on the go not too bad. At first the custom manifold we made didn't allow room for air filters, so mesh it was for a while. I dropped the clips to make up for this.
Over winter while off the road, the manifold was modified to accommodate room for filters. Started up and ran about as well as before.
One thing I did have an issue with is the odd bork when cornering harder to the left. The carbs are mounted on the car in line from rear to front with air intake to offside (cyl 1 is at the back of the car). This happened the other day to the point where she cut out. I think I may have flooded trying to re-start, and being towed an hour or so later, tried to bump it and got only a couple of big misfires. (Could this have further hindered things).
Since then, I've checked I have spark, got fuel to the carbs and got compression. With the carbs off I can see the accelerator pump is working ok. Not sure how I can check the main jets. However when turning over, I don't think fuel is reaching the cylinders. The plugs aren't getting wet, even when really turning over for a while.
I did briefly get it to make a little "hissy cough" when turning over a few times when giving a brief pump of the throttle prior to turning over.
I'm not any sort of carb expert, but I was wondering if there is anything on the carbs I should be paying particular attention to. Plus does anyone ever have an issue with borking out? Though my case was left hand bends, on a bike it would be the equivalent of a hard stop!
In any case I hope this makes sense. Thanks in advance for any kind input / info that can be provided.
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Interesting conversion! Many years ago I owned a Bond Bug, and we fitted CB900F carbs to it, went like the clappers, and even gave 70+ to the gallon! Truthful answer is I always advise anyone with K7 or F2 carbs messing about to dump them and fit carbs from another CB750 model. The PD carbs are well known for clogging up internally due to the tiny internal passageways, they are okay (acceptable) when spotlessly clean and well set up, but the tiniest bit of crud in them is a nightmare. I would certainly get them stripped and thoroughly cleaned again, and consider putting a diesel filter head in the fuel line (The sort that are generally bolted to the bulkhead of diesel Landrovers) should cost peanuts at a scrapyard, and the filter elements are relatively cheap and long lasting. Most likely cause of crud getting in the carbs, particularly when cornering hard is the fuel in the tank sloshing about and washing particles of rust from the inside of the tank into the fuel line. Did you clean or seal the tank before doing the conversion? Or has it got 40-50yrs of rust and scale floating around in it? :o Also, bike carbs are made for gravity feed (Except for a few oddities like Goldwings) and the fuel pump pressure may be overpowering the float valves, particularly if it is a mechanical pump rather than electric. Remember to flush the fuel lines through as well, otherwise the whole lot will need stripping and cleaning again. :-[
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I bet the bond was fun!
Thanks for the reply.
I've got an alloy foam filled fuel tank so that should be well baffled. I try to keep it full too. All new fuel lines (though rubber), running with a facet electric fuel pump with small inline filter, then in to a larger filter king fuel pressure regulator and filter. Pressure should be ok as when it was too much, I got fuel from the drains.
Will take them off and check all of the passages are clear. Do you know if any other 750 carbs will fit straight in to the inlet manifold (size and spacing)? The carbs are held on by silicone hoses so a few mm either way will be ok!
Cheers
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In that case, any bank of CB750 carbs can be made to fit, as only the intake rubbers need changing on a K7 or F2
Sent from my X6pro using Tapatalk
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That's great news. Sorry to ask further, but is any particular year better than another? I.e. For setting up or reliability etc.
Trust me to have picked up the worst ones. I knew I was in for trouble when I found out they only used them for one year!
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Not really that much difference, mainly jet sizes etc due to ever tightening emissions laws. Irrelevant really, as you will want to jet to suit the Imp engine, you may need to use a bit more throttle to pull away due to the lack of accelerator pumps in the other models, but Imps are like Honda's, they thrive on revs. K3/4/5 can be bought relatively cheaply from places like D.K. spares as they import a lot of bikes from the U.S. and break a lot of them.
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Brilliant info, thanks again, it really is appreciated.
Just an update, got the carbs off last night, and took the float bowls off. Attached some tube to the mains one at a time, and blew up, each appears ok.
Do the same with each of the smaller (pilots/idle?) and they are all clear.
Blew through the fuel feed, and floats are all working ok. Wonder if I have unblocked something in the process.
What I did find was a couple of pin holes in the inlet manifold, and that at the inlet on no1, there was light showing through the gasket when I shine it through, maybe a bit warped. This will not help at all. I will address the manifold next, then pop it all back together and check the timing. Not sure if / how that would have gone out but worth a check.
Thanks again, Gav.
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The manifold problem could probably be the culprit. If its not totally sealed it will draw air in and lean out the mixture. That will cause the baulking that you talk about.
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I'm on with it now. I will get there!
Gonna try getting something down the small ports (2 little pipes) on the air intake suddenly of each carb (not sure what they do mind).
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I wouldn't disagree with the above but hope to add a little to help.
As hairy pointed out, many don't like the PD carbs as if they stand around not being used much they can give problems related to blocking of the small jets and require cleaning to get them working properly.
I like them though and if given a clean supply of fuel and regular use they are nicely accurate carbs I feel.
Saying that, they are fussy and difficult to alter, even on their original engine if you modify the intake system. I don't know what you've got in the way of filter hardware but they would originally have had bell mouths fitted inside the original airbox. I believe these are vital to making them run properly and think you'd be very compromised without them. Also, I feel they benefit from an airbox/plenum even if it was to be a small one supplied by a large bore intake hose leading to a remote filter if it can't be fitted direct to the carbs.
The fuel supply for a car I'd consider a fuel return system in which you go inline from the tank, pump, filter, then T off to the carbs but continue the fuel line into the pressure regulator and onwards back to the fuel tank. This will give you a pressure regulated supply to all the carbs inlet valve / float chambers as opposed to effectively coming from the current system with regulator before the carbs and any fuel after that point being variable in pressure and quantity delivered to each carb.
But most fundamentally I think you'll run into setup difficulties with this particular carb setup as it maybe giving you a mismatch that's hard to get out of.
I'll put a further post to cover that statement.
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My understanding of these carb sets is as follows.
These PD series were specifically designed to run as lean a mixture as possible through lower and mid range throttle positions, to a degree that the motor will be too lean to accelerate cleanly. This is what is facilitated by the "accelerator pump" function to get it going just as you are winding the throttle slides open.
The advantages of running lean are that under low or light load the mixture is burnt more completely, giving less unburnt fuel to pass the piston rings or into the exhaust and out as emissions. This compared to a normal jetted carb that has to accelerate cleanly without additional fuel so in effect running slightly rich all the time. Benefits of less oil contamination and less unburnt fuel in the exhaust system will just make the components last longer.
I'd estimate that with the bike use it would see more throttle openings than in a car just because of having five closer gear ratios, so more times the fuel fuel is supplemented with the pump as you close then open the throttles more frequently.
The Jetting at wide open throttle has to be more close to ideal (not lean) as when held wide open under maximum load it can't be supplemented by the pump, so you get the throttle slide cutaway and needle taper designed to give you a fuel gradient that goes from pretty lean at bottom to middle, up to fully supplied at the top end. It's this relationship I don't believe you can readily change when trying to set these carbs.
What people do is respond to the lean running by lifting the needle or fitting a bigger mainjet which increases fueling across the range. So you may get the bottom and mid somewhere near to what you want, but then the top is overfuelled and runs rich.
I don't know if you can resolve this, or certainly not without a dyno run with exhaust gas temp and analysis to see where you are.
If it were me making a decesion, I'd go for that choice that Hairy has given above with the CB900F carbs as a much better starting point.
It's a much closer starting point I feel regarding engine capacity and not a bad match to the Imp motor being able to flow for that capacity (what size are you running?) as they I think originally had a rev range to about 10,000rpm with bhp around 96.
They came from the development program of Honda's RCB endurance racers and where specifically targeted at good starting over any heat range, economy and particularly well resolved response over the whole rev range. You may even find they are, even with no adjustment, very close to the demands the Imp motor requires.
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Crikey, that's the most complete answer I have ever seen on a forum. Nice one!! Also helps us F2 boys understand the system a little better. Thanks.
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Yeh Roy, I don't think you'd want to get stuck talking to me at a party! :) specialised interest, lean burn carbs fitted to Honda motorcycles of mid to late seventies. ;D
I do think they are very good carbs though if they are properly taken care of.
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Absolutely invaluable information. All makes more sense now.
The engine was originally 875cc, but I have used a larger Talbot sunbeam 930cc unit for the slightly bigger capacity and stronger block. The next steps are boring out to 998 or 1040 so the 930 is a happy (ish) medium from standard 875.
I will try to add a photo of the set up. At first, the intake side of the carbs was so close to the offside chassis rail that I could only get some mesh (cut up sieve) jubilee clipped over each carb inlet. After reading some forum advice regarding the extra air flow, I lifted the needles on position on the clips.
Over winter I had the manifold modified some more to allow room for better filters. All I could fibd was some cheapo k and n style copy things on eBay. They do the job but one jubilee clip has already breached its rubber connector. I also put in different main jets. Will have to check out the size.
I will look at the 900 carbs as an option. So thanks again for that advice.
The main issue is my air leaks. I have just dropped the manifold off with the welder, he's hopefully sorting it Monday, then I can get the face of the flange skinned. It is warped by about 1mm at either end.
I must admit, as much as these things annoy I us and cause you to tear your hair out, it keeps things interesting!
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Yeh Roy, I don't think you'd want to get stuck talking to me at a party! :) specialised interest, lean burn carbs fitted to Honda motorcycles of mid to late seventies. ;D
I do think they are very good carbs though if they are properly taken care of.
I think that would be a subject on MASTERMIND ,
with no passes !!
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A couple of pics (sorry I know its not a bike!
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And of course what the Honda technology pushes along...
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Johnwebley, there may not be much of an audience for it though. Also in a competition for Honda general knowledge I'd be soundly beaten by some of the others on here.
Zippy, the Imp looks nice and is after all a SOHC so it scrapes through ok on that front. Must be one of the most diverse motors around though as I've seen them in both solo motorcycles along with quite a few road race outfits in the past, this along with their four wheel applications of track and extensive rally use must cover many forms of motorsports.
Seeing the installation like that and proximity of the chassis leg got me thinking more about suitable carbs. If you take a 900/1000cc bike motor as a good source, then something like the original Yamaha 1000 Exup has a similar potential to the Honda 900 carbs but the whole bank of carbs is tilted up at 45 degrees as the bike cylinder install was lent forward at that angle. This means the carb intakes would possibly be above and clear of the chassis in your use so may offer something useful in the way of suitability.
Thinking further about your setup as it is, it may be realistic to make up some bespoke needles for the carbs with a slightly smaller diameter at the thick end of the needle to bring the lower throttle opening mixture a little closer to the topend to give a more linear progression to work with.
As you say, it's interesting to work through something like this anyway with the satisfaction if you get get good progress.
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Crikey, that's the most complete answer I have ever seen on a forum. Nice one!! Also helps us F2 boys understand the system a little better. Thanks.
;) yes them PD carbs are a learning curve i have a set on my k0 sidecar combination and have often thought of sticking k1 or k2 carbs on instead,they work quite well for me for normal riding but often it cuts out on tick over probably due to over lean mixture. the thing that has always puzzled me with these PD carbs is why the accelerator pump only works on one carb and cylinder ,why not have four one for each carb lol
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So I'm back up and running. Leaks all fixed.
The problem was.... a crappy rotor arm that had spun itself out of position inside the dizzy. Nightmare!
Anyway I'm back on with setting the carbs up now. Bit of oily type residue on 3 plugs but good on the others. Think this may be down to no valve stem seals as opposed to a carb problem.
Need to get hold of a balancer, but all in all she's going well.
Thanks again for all of the kind help and advice.
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Crikey, that's the most complete answer I have ever seen on a forum. Nice one!! Also helps us F2 boys understand the system a little better. Thanks.
;) yes them PD carbs are a learning curve i have a set on my k0 sidecar combination and have often thought of sticking k1 or k2 carbs on instead,they work quite well for me for normal riding but often it cuts out on tick over probably due to over lean mixture. the thing that has always puzzled me with these PD carbs is why the accelerator pump only works on one carb and cylinder ,why not have four one for each carb lol
Steff , the accelerator pump it mounted on one carb but pumps fuel into all four.