Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Liam on February 15, 2018, 08:31:07 AM
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Hi
Nearing the end of my build and one of the last tasks is to sort the jetting out Anyone recommend a dyno tester who could sort the jetting out on my bike... usual mods which I inherited - pod filters and 4 into 1 exhaust. I'm in the Saddleworth / East Manchester
Done the research and seems like a mare to get spot on...
The bike is a CB750F Supersport and I'm in the Saddleworth / East Manchester
Cheers
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[attachimg=1]Hi Liam, I’m not local to you so I don’t know of any dynoes in your area, I have a K6 engine in mine with PD41a carbs/pod filters and a 4 into 1, so it’s similar, I guessed my main jet sizes, went with 135 mains much bigger than the standard jet size, standard primary jets, needles on the middle notch, cheap Chinese pod filters , took it to Sam ermelenko’s dyno in Tutbury, fuelling was spot on, it ran great apart from it “surged” a bit when trying to cruise at a steady 70mph, I suspected it was the airflow into the carbs, if you look at the rubber bungs that hold the pods to the carb they have a square edge, the standard air box has 4 rubber velocity stacks, so I took the rubber velocity stacks off the original air box , removed the rubbers from the pods and grafted the rubber stacks in their place, problem solved, runs great through the range now and holds steady at any cruising speed, Honda didn’t go to the trouble of fitting the rubber stacks for no reason so my guess is they smooth the flow into the carbs and this aids carburation, I can’t find a photo of the mod I did but if you search on the forum there will be one in one of my threads. EDIT, you can see them on this photo .
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Just wondering Mark, how brutal were they at the dyno...... I went to look at a dyno test a couple of years ago (not my bike thankfully) and they revved the bike hard through all the gears...... The engineer said afterwards that you have to sign a disclaimer and they blow-up engines on a regular basis....... There was a large air fan in front of the dyno, but as they revved it out in top gear it should have been doing 120mph and the gentle breeze from the fan just did not feel enough to cool the engine. I don't know enough about dyno testing to say that it was a daft thing to do on an old road bike, but their not doing any of mine. I know you need to give it the beans to test it properly, but is this the normal thing or was it just this guy that was hard on the machinery.
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I have been with Ralph Holland at MK Dyno a few times when he has blown up the customers engine in front of them. The customers face, when Ralf says, " well, that's what you just paid me to do" ;D ;D ;D To dyno any engine it must be at its best in the first place ;)
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Thanks gents,
Roy, Trigger - very good point regarding full throttle through the range. Unlikely I'll go through with it.
In the meantime Mark - I think I'll fettle the the main jet size and see how it runs / how the plug look. Needle is on the middle setting, seems to tick over okay, no popping etc. but still have to balance the carbs. You're right about the velocity stack in the stock air box... Good ol' "Hondaman" gives an insight into the flow of air being patterned to prevent flat spots between the low, mid range and full throttle. Unfortunately the bike came with Chinese pods which were split etc. I swapped for "ramair" filters so no doubt will be dealing with at least one flat spot.
Cheers Gents
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Just wondering Mark, how brutal were they at the dyno...... I went to look at a dyno test a couple of years ago (not my bike thankfully) and they revved the bike hard through all the gears...... The engineer said afterwards that you have to sign a disclaimer and they blow-up engines on a regular basis....... There was a large air fan in front of the dyno, but as they revved it out in top gear it should have been doing 120mph and the gentle breeze from the fan just did not feel enough to cool the engine. I don't know enough about dyno testing to say that it was a daft thing to do on an old road bike, but their not doing any of mine. I know you need to give it the beans to test it properly, but is this the normal thing or was it just this guy that was hard on the machinery.
Hi Roy, I was in the dyno room with the tester, I had already said be careful it’s an old engine, he said don’t worry I’m not going to thrash it, we agreed not to go beyond 8.5k revs, it was really no different to briefly gunning it on the road, just make sure that they know what you expect, and be with the bike if they’ll let you, cheers Mark
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There's an obvious concern with the heat accumulated during these runs but as pointed out, you should have it in good enough mechanical condition to complete it without bringing up significant problems.
Getting a general check of jetting through the whole range should be the first objective to make sure it's running within range to reduce risk, any significant errors you'd have to correct, or suspend running.
One thing that's normally not considered is plugs. You need to be using the hottest that you can get really to guard against overheating them, leading to pre-ignition. That'll blow it up faster than you can close the throttle in extreme cases. Starting with at least an NGK 9 heat range, or even a 10.
Purely mechanically, the rotating bits should cope with the revs (if not it'll probably go on the road anyway) that's if it's put together properly. Also, synthetic oils will cope with this heat range better than non.
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Hi
Nearing the end of my build and one of the last tasks is to sort the jetting out Anyone recommend a dyno tester who could sort the jetting out on my bike... usual mods which I inherited - pod filters and 4 into 1 exhaust. I'm in the Saddleworth / East Manchester
Done the research and seems like a mare to get spot on...
The bike is a CB750F Supersport and I'm in the Saddleworth / East Manchester
Cheers
Hi Liam,
I have just replied to private message, sorry for the delay I have not been on this Forum for several weeks
I hope you find the info use full, as I have mentioned in the private post Pod filter are not good enough you will have some issues as certain revs as they don't let enough air in, you need velocity stacks from Dime City USA about £90 including all taxes, since putting these on my bike it runs perfect.
The other thing is as other here have mentioned here forget the Dyno
good luck & keep me posted
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They look like the same pods I have on mine had a devil of a time trying to get it to run properly till I realised that the mounting rubbers actually reduce the intake too 28mm from 38mm so I was getting over rich mix pulled the rubbers out mad some new mounting out of polypipe and injinuity the differance was remarkable! Now just saving pennies till can afford descent set up.
Would like these guys to set it up for me but that's a dream.
http://www.d-mengineering.co.uk
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posted by Colonial_clive
SOHC Member.
Would like these guys to set it up for me but that's a dream.
http://www.d-mengineering.co.uk
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I know Dave at D-mengineering.. he is the best engine builder in the country ! but very expensive, but not only that he uses a Dyno company at Newark Torquetune the owner & guy who does the Dyno is called Russ....Not Recommended believe me, he is the same one that I used & he buggered up the carbs on my CB 750 big time & I had nightmare & more expense getting them sorted after he had done with them ...... Honda CB 750 / 550 + other classic bike owners Stay clear of Russ at Newark Dyno...
if you have a modern bike & just want a power commander fitted then that's just about his bag >:(