CB750 DOHC and Keihin CR31 settings

Logolibre

New Member
Hello, everyone.

I have a Honda CB750 Bol d'Or motorcycle (1982, DOHC, RC04) with Keihin CR31 carburetors, zero resistance filters, straight-through exhaust (4-2) and Dynatek 2000 electronic ignition installed. The engine itself has no tuning.

The problem is a rich mixture on all modes. But it is especially noticeable at idle. The motorcycle starts even in cold weather without a choke. The spark plugs are quickly covered with soot and start working badly.

The jets are standard, the main jets are 120, the slow jets are 60. And it turns out that's a lot. I couldn't tune it with what I had.
Before ordering smaller jets, I would like some advice. Maybe it's not about them at all?

I've heard some feedback on the web that people ride fine on this set, bike runs great, etc. The dealer also wrote that everything should work fine.

Anybody want to share their experiences? Including the size of the jets.
 
get a heat gun and verify that all cylinders are firing dohc can be a electrical nightmare so make sure both 1-4 and 2-3 are firing
as if its running on one set weakly it will look like the carbs are rich
i go through this with clients with dohc almost daily

98 percent of dohc carb issues are electrical in nature take temp readings they will help you pin point issues
 
get a heat gun and verify that all cylinders are firing dohc can be a electrical nightmare so make sure both 1-4 and 2-3 are firing
as if its running on one set weakly it will look like the carbs are rich
i go through this with clients with dohc almost daily

98 percent of dohc carb issues are electrical in nature take temp readings they will help you pin point issues
Thx for answer!


This Honda is already becoming a nightmare for me :)

Again, I have a Dynatek 2000 electronic ignition, Dynatek ignition coils and Keihin CR31 carburetors with jets: main - 120, pilot - 65.

The ignition was set according to the instructions, last season I ride on this setup, the motorcycle did not work stably everywhere, there were problems with idle, but in general it was ok. At least not that much gasoline was pouring, there was no hellish smoke from pipes and the smell of unburned gasoline.

I tried to put other pilot jets - 42 sizes. The motorcycle stopped smoking completely, but the idle speed stuck in the region of 2500-3000, and did not change in any way. When over-gasing, the revolutions are reset with a delay.
And now the motorcycle has completely ceased to start from the word at all.

The ignition is working correctly (it seems)
There is a spark on all candles
Gasoline enters the carburetor (the chambers are filled) and the cylinders (the candles are wet)
Engine turns normally

But the motorcycle does not show any signs of life at all, does not even try to start.

So far, the theory is as follows:
- the candles are out of order (there is a spark, but bad)
- possibly! incorrectly set cam timing, from this problem, incorrectly set ignition, etc.

I will sort this out. But, if someone can push you on the right path (what to do, what to check) - the buddha is immensely grateful :)
 
"Zero resistance filters"? You might check the DOHC site to see what they suggest. A lot of experience there.
 
"Zero resistance filters"? You might check the DOHC site to see what they suggest. A lot of experience there.
Yes. But on the engine install flat-slide carbs, not stock CV carbs. And there shouldn't be a problem with zero-pods

Another theory has appeared - air leaks. Either through the gasket or a crack in the cylinder.
 
when one does that for me the first thing i would do is put a new set of plugs they can carbon track real quick

also verify your compression to rule out basic mechanical issues
 
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