Keihin CR29 on 500 or 750.

Hi,

I found a set of used CR29s in Japan from a Z2. 

Plan was to use it on my Z500 but I thought they might be too big. 
I have a set of Mikuni VM24 from Z650 that I brotched to 26mm and modified to fit the 500 engine
 

So the idea now is to put the CR29s on my Z650. 
I have two engines for it. One 738cc and one 810cc.
Is CR29s to small for the 738/810 engines?
The engine specs are similar to 1984 GPZ750 but a little more compression and 4-1 exhaust.

Should I try to swap them for a set of 31 or 33s?
Power between 6000-11000 is what I’m looking for.

Thanks
Sixten

Sixten wrote:

Hi,

I found a set of used CR29s in Japan from a Z2. 



Is CR29s to small for the 738/810 engines?


Thanks
Sixten

No! CR29's are suitable for 650 to 1000cc.

I will tell you a couple of things about those CR’s before you dive into them. They are a real pain to get dialed in. I myself have the 26’s on my 550 and I had to go through some pilot and main jet changes to get it right. Right out of the box, my pilots were way too rich had to step down from 65 to 55s and I had to go up a couple on my main. The choke is horrible in these, might have to go down some sizes with that as well since they’re so damn rich. Also for the idle you’re going to have to set it to around 1500 or else it’ll pretty much stall. Could be different with yours, but that was my experiences with the CR’s.

Sixten post=906800 userid=57394
So the idea now is to put the CR29s on my Z650. 
I have two engines for it. One 738cc and one 810cc.
Is CR29s to small for the 738/810 engines?
The engine specs are similar to 1984 GPZ750 but a little more compression and 4-1 exhaust.

Should I try to swap them for a set of 31 or 33s?
Power between 6000-11000 is what I’m looking for.

If you don't have a crazy good professionally ported big valve cylinder head on the 810cc engine, stick with the 29's as they flow plenty for a stock GPZ head. If you want power in that rev range maybe look for some higher lift cams to take advantage of the bigger GPZ ports.

Those GPZ heads really come alive with a cam in the .380-.400" lift range.

I talked to Larry C about porting a GPZ750 head and he said it only requires slight cleanup/port match at the manifolds to be excellent. He mentioned really taking material out is a big mistake flow wise especially for street/road race purposes as velocity suffers (flow bench proven) He said he would only enlarge one if it was for drag racing.

There was a guy on here from Italy that road raced a 810 engine, he welded up the combustion chambers! Just search "turboguzzi" -  he ran CR29's

Thank you for the advice. I made new spigots for the CR29s to fit the 750 head. Tapered from 29 to 33 to match the diameter of the intake ports.
The 29s will probably give better control in the midrange than 31/33s. 
That’s a good thing I think because I plan to also try Flat track racing with the 750 next summer.

 I tested the CR29s on the 650/750 now and it felt strong. Difficult to say if it makes more power than BS34.
When I fullly open the throttle on mid RPM it runs a little bit lean before picking up the revs and eventually shows rich on the AFR gauge.
Maybe its normal that a large racing carburetor runs lean when opened fully at mid RPM?
Also the idle is around 3k RPM and I cant get it lower. I don’t know what that can be?

Holy Toledo!

Sixten wrote:

Also the idle is around 3k RPM and I cant get it lower. I don’t know what that can be?

 

What else could it be other than extra air passing into the engine, either through the carbs or some other sneak path? 

Any chance you have an air leak between the carb boots and head, or at either end of the spigots?

Have you checked that the throttle slide stop is set correctly, and that the throttle cables have enough travel to fully close the slides?