Hello ladies an gennul men. I am back working on the cb750 chop again in anticipation of next summer. It’s a standard 1978 model engine in a chop frame, it has a straight through pipe with little cone breathers on the carbs. Now, on tick over in first, I can Poodle round the car park no. Other, when you go to ring its neck, you get to about 1500 rpm and it chokes itself out completely. No I know this will be an air fuel mixture issue. What I want is a helping hand on setting the mixture correctly, or even a contact for someone who can undertake such a boring task. I’ve heard the, cv carbs won’t work with pods, put a standard pipe on it… Blah blah shhhh, it’s a chop, standard is evil, it must be removed. Any help would be gratefully received, thanks again girls and guys,… And merry bloody crimbo to the lot of ya, you’re all ace, apart from smiled, he sucks balls., love ya shaneypoos.
Ideally you want to get it on a dyno machine so you can see if its too rich/lean and get them to set the mixture,
Pole position in elephant and castle were doing dyno runs for £10 on a Saturday (not sure if there still doing it tho.)
Riverside dyno are in old ford but I can remember there prices…
When I put the same type of air filters on my bike I uped the main jets from 95 to 122
Easy job to do but still get it on a dyno to check
I also think its better to be a bit on the rich side
if you are resigned to running a straight through pipe and stupid filters it will never run right. however, take it to PDQ in taplow and they will make it run the best it can for what it is. not the cheapest guys, but you get what you pay for.
+1 PDQ are the nuts draw for tuning . You can get a four running on straights and pods but midrange becomes weak which is unfortunate as a chop wants to have midrange . Really ya should have got a twin if you want to do that , as you can get away with it on them much easier .Something to do with surface area of piston vs valves . Often with a twin if you stick a couple of washers on the needles it will run fine .
In theory (you have to love that as an intro.) lowering the resistance of the intake air can be “fairly easily” dealt with. You may have to change jets, needles, settings and the like but it is doable.
Straight through pipes cause problems. It’s to do with the lack of back pressure in the system, but, even if I could remember what all that was about it would take forever to explain.
You might be lucky and get it right on your own one day, but my advice would be to find someone with a dyno. that does this for a living. It can be a bitter pill to swallow but by the time you have spent £lots and time lots on bits and pieces and still not found the sweet spot, it’s not so bad and a lot less grief.
Come on then matey let’s see what it looks like!
Hhahahaaaa, thanks for the tips, it is technically a straight pipe, I thought I lost the brass baffle, but after a garage rummage looks like it is present,woohoo for back pressure. Will do Conrad, I’ll bosh some pics up later once I work out how to use my first iPhone!!! Bloody Luddite!!.
+1 for pictures! would love to see what you are creating. (I’m going for a chop in the new year too). Interesting thread…
I promise to at least attempt to upload pics, I am a BLOKE with a COLD and a new IPHONE, the odds are stacked considerably against me. As it happens, I’m also looking to swap the flat rear mud guard for a round one, anyone got any bitz kickin aboot!!!