Author Topic: No Vacuum on Carb#1  (Read 629 times)

Offline element137

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No Vacuum on Carb#1
« on: January 11, 2021, 09:10:38 AM »
Hi all - been a lurker on here for a while- this is great resource -many thanks to all for the info - its a massive help - currently restoring 76 400F and have hit a problem with trying to sync the carbs as I cant get a vacuum on Carb#1 - background so far - apologies for long post -
Bike was a non runner when I got it late last year - got it running but very rough running, wouldn't idle -
Stripped carbs completely , very badly gunked up - ultrasonic clean and vapour blast - reassembly - ( have used aftermarket brass except for float valve where I used the original OEM valve ) - first attempt with aftermarket float valves resulted in leaks on all carbs -
So this week tried to sync carbs using vacuum gauges - carbs 2/3/4 all pretty much spot on from bench sync - but no vacuum on Carb 1 - can move the gauge if I suck/blow on the line - so not the gauge -
I have sprayed Carb Cleaner around all the inlet rubbers and carb tops whilst its running which has had no effect - no rise in revs or stall - so I don't think there is a leak at the inlet / carb tops
Header on cylinder one just warm to the touch where others are hot from running - so I guess its not running on cylinder 1 - couldn't get the spark plug out yesterday so have not checked if its getting a spark - but my question is -
is there anything I could have screwed up so badly rebuilding the carbs that would cause vacuum loss on one carb ? - also noted that is starts from cold without choke - so assuming its too rich, also idle screw is turned out so far its no longer in contact with the bar at all - I am hoping I have messed up the carbs/bench sync rather than it being a bigger problem - but could that cause no vacuum on one carb in isolation ?

Offline Johnwebley

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Re: No Vacuum on Carb#1
« Reply #1 on: January 11, 2021, 09:34:00 AM »
Have you checked the valves have  clearance and are opening and closing properly?

Use another plug to check the electrics work. Lay it on the head, see if it sparks.

Undo the carb drain to make sure fuel is in the carb

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Offline element137

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Re: No Vacuum on Carb#1
« Reply #2 on: January 11, 2021, 09:52:49 AM »
Thanks - the valve/spark check is my plan for Friday this week when I have a day to spend on it - I'll open the drain to see if there is fuel - so hopefully can start to eliminate some of the variables - as I have had the carbs apart its likely I might have messed up - I was hoping that someone might recognise this issue -   and point at the fact I might have done something dumb and obvious on the carb rebuild/bench sync - its clearly running very rich ( no choke needed  ) and the idle screw position means that in effect, its redundant now - so I am thinking I have messed up the rebuild of the carbs -

Offline ka-ja

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Re: No Vacuum on Carb#1
« Reply #3 on: January 11, 2021, 11:54:45 AM »
The most obvious thing after valve clearances, is to check out the "non-removable" spark plug, I have had a faulty (brand new) spark plug before giving those exact same symptoms.
nice bike,nothing in the bank

Offline MrDavo

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Re: No Vacuum on Carb#1
« Reply #4 on: January 11, 2021, 12:30:08 PM »
I've tried to do the valve clearances on the wrong stroke before, so there's that to check.

The plug that won't come out is worrying. It could be cross threaded - I've read about plug thread helicoils being done in situ, with something to catch or remove swarf from the combustion chamber without dismantling, but I've never tried it.
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Offline hairygit

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Re: No Vacuum on Carb#1
« Reply #5 on: January 11, 2021, 12:36:47 PM »
You bought it as a non runner. I guess you haven't done a compression test? If you have, what were the readings on cylinder 1? I hate to be the harbinger of doom, but if you bought it as a non runner there are so many things it could be. Damaged/ bent or siezed valve, holed piston, knackered rings/ bore. So first thing, get that plug out and and hold your finger over the plug hole while winding it over or kicking it over with throttles wide open. If it doesn't blow your finger of the hole, it's going to need further investigation.

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Offline element137

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Re: No Vacuum on Carb#1
« Reply #6 on: January 11, 2021, 01:58:36 PM »
Just to clarify couple of points - I've had it running before, although very briefly and very rough -  and I am pretty sure cylinder 1 was running OK (although not long enough to get any real temperature into it) - when I said I couldn't get the spark plug out it's because I didn't have a socket/box spanner - so as far as I know it should come out when I get a chance - apologies for the confusion - I have ordered a compression gauge so I can check all cylinders anyway as a general health check - lesson learned - should have done it before taking carbs off etc -

Offline Johnwebley

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Re: No Vacuum on Carb#1
« Reply #7 on: January 11, 2021, 03:12:00 PM »
I suggest you get a replica Honda plug socket.

Not expensive. But invaluable

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Offline robvangulik

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Re: No Vacuum on Carb#1
« Reply #8 on: January 11, 2021, 03:26:15 PM »
I have ordered a compression gauge so I can check all cylinders anyway as a general health check - lesson learned - should have done it before taking carbs off etc -
That is one thing you can do very well without carbs, at least you can't forget then to open the throttle fully ;D

Offline ph0010421

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Re: No Vacuum on Carb#1
« Reply #9 on: January 11, 2021, 10:14:07 PM »
Oddly enough, I had no vac on cyl 1 doing a synch this weekend with my cheap 'red box' guages.
Reason...there was no hole in one of the extension tubes. I also thought the guage was fubar.
Other than that (there's loads of different tubes included, so no great shakes) the precision was amazingly good.
« Last Edit: January 11, 2021, 10:16:16 PM by ph0010421 »
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Offline element137

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Re: No Vacuum on Carb#1
« Reply #10 on: January 12, 2021, 02:28:30 PM »
sounds like a similar bit of kit to what I have used for the sync - I did think that the adapter on Carb 1 might be dodgy so swapped it for another one from the kit and no different - on carbs 2/3/4 they worked a treat and they were pretty close from bench sync to start with - just Carb#1 gauge not registering any vacuum -

Offline Rob62

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Re: No Vacuum on Carb#1
« Reply #11 on: January 12, 2021, 07:39:24 PM »
Sounds a bit worrying, if there is no vacuum that means there is insufficient movement or air through the cylinder which could be damaged, bent or burned out valves or missing, broken or stuck rings or a holed piston.... you should be able to get an early indication of the situation by putting your thumb over the plug hole and turning the motor over.

Offline element137

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Re: No Vacuum on Carb#1
« Reply #12 on: January 15, 2021, 11:06:24 AM »
Update : Cylinder 1 compression OK - and getting a spark - turns out the lack of vacuum on CYL 1 when using 4 bank carb sync gauges = blocked adapter  :-[ - last week I only tested the gauge by sucking on the tube - swapped the adapter over and got vacuum on cylinder 1 OK - so down to cheap crap vac gauge set - lesson learned - combination of cheap gauge and my dumbness -
Had it running this morning - idle is a bit erratic with a bit of surging - but now CYL 2 intermittent not firing - pulled the plug - and intermittent spark/weak yellow - any ideas on this ? coils ? - if I remember the coil on 2 is paired with 3 ? which is running OK - its running what looks like a very old Piranha electronic ignition -

Offline Bryanj

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Re: No Vacuum on Carb#1
« Reply #13 on: January 15, 2021, 12:17:01 PM »
New plug caps, trim the ends of the HT leads and try again

Offline element137

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Re: No Vacuum on Carb#1
« Reply #14 on: January 15, 2021, 12:53:26 PM »
Thank you Bryanj - I followed your bench sync process ( thank you very much for that ) - as I mentioned in prev post its running very rich, starts without choke, and the idle screw is screwed so far out that its no longer in contact with the bar - I am just about to pull the carbs off again as the idle is rough/surging (as well as the spark issue on CYL2)  - in your opinion - with the idle screw in this position ( ie, redundant ) , is it possible i have screwed up the carb rebuild please ? apologies if its a bit of a how long is piece of string question - I don't really want to take the carbs apart again if I can help it - but I am sort of convinced something not right ? - could the over rich issue be the air / mix screw ? - currently set all four at 1.5 turns out and untouched at this point -appreciate any thoughts

 

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