Topic: Getting Ready To Dyno Street Build CB750K ATP Turbo

[teamtiger21]

Wrapped up a couple weeks ago the freshening of a CB750K ATP Turbo build from the 70's. I want to arrange some dyno time for the tune and pop off adjustment. Considering setting at 6-8 pounds. Engine has Carrillo rods and turbo pistons, Zenith carb. Key question is, do I need to run a 3-4 pound pressure fuel pump? I am getting conflicting information for a Honda street application. Thanks Barry

[Cb750 Racer]

Wrapped up a couple weeks ago the freshening of a CB750K ATP Turbo build from the 70's. I want to arrange some dyno time for the tune and pop off adjustment. Considering setting at 6-8 pounds. Engine has Carrillo rods and turbo pistons, Zenith carb. Key question is, do I need to run a 3-4 pound pressure fuel pump? I am getting conflicting information for a Honda street application. Thanks Barry

6-8lbs of boost should be fine with that build sheet assuming the compression is stock or lower. The Zenith carb is very primitive you'd get much betting drivability with a newer style carb like the Mikuni HSR42. I also completely understand if you're trying to make it look period correct as well I can appreciate that.A fuel pump is some nice added insurance I'd recommend one 100% these kits came with one back in the day so it definitely shouldn't be overlooked.

[teamtiger21]

Thanks Racer, yes I am trying to keep it period. I have a couple of late 60's builds that don't run one. My original packing list does not have it listed. One concern is will Zenith hold the pressure. Zenith bowel is small, close the petcock and it runs out very quickly. Don't want to get on dyno to find I need one. Barry

[PeWe]

Really beautiful build!

[MRieck]

Thanks Racer, yes I am trying to keep it period. I have a couple of late 60's builds that don't run one. My original packing list does not have it listed. One concern is will Zenith hold the pressure. Zenith bowel is small, close the petcock and it runs out very quickly. Don't want to get on dyno to find I need one. Barry

3 to 4 lbs of fuel pressure is a lot for a carb.

[Cb750 Racer]

Thanks Racer, yes I am trying to keep it period. I have a couple of late 60's builds that don't run one. My original packing list does not have it listed. One concern is will Zenith hold the pressure. Zenith bowel is small, close the petcock and it runs out very quickly. Don't want to get on dyno to find I need one. Barry

I appreciate you keeping it period correct it really is an amazing looking bike! If your kit has the log style manifold then it is very possible that no fuel pump was included when it was purchased. I believe they made fuel pumps standard on the later version of the kit with the "Spyder" 4 into 1 exhaust manifold and for good reason. Like MRieck said 3-4 pounds is a lot of fuel pressure for a carb it's really just to make sure you have a consistent amount of fuel in the bowl.

[teamtiger21]

Thanks Racer for that referral on the photo of a later ATP kit. Yes mine has much different manifolds. Looks like the kits that i'm familiar with that don't have pumps are all early. The tune will be for limited street use, so I will give it a shot without a pump.

[scottly]

3 to 4 lbs of fuel pressure is a lot for a carb.

Back when cars had carbs, and mechanical fuel pumps, the normal fuel pressure was about 5-7 PSI.
With Webers, lower pressures, like 2.5-3 PSI, were recommended. I ditched both the electric pump and pressure regulator 20 years ago on my bike, and rely on gravity now.

[MRieck]

3 to 4 lbs of fuel pressure is a lot for a carb.

Back when cars had carbs, and mechanical fuel pumps, the normal fuel pressure was about 5-7 PSI.
With Webers, lower pressures, like 2.5-3 PSI, were recommended. I ditched both the electric pump and pressure regulator 20 years ago on my bike, and rely on gravity now.

  I was thinking of the more modern carbs I would see on bikes in the late 80's/90's...Shadows, VFR's, SV's etc. Just enough pressure keep the bowls full

[TurboD]

If you are only planning on running 6-8 psi of boost, you may not even need a pump as long as you have a good flowing petcock and line.

The old Bendix Zenith does not like much fuel pressure, 2-2.5 max. The last turbo Honda I built, I used a fuel pump for a Honda Shadow which has about 2 psi that I bought on Feebay.

[teamtiger21]

Thanks Turbo, yes petcock has been drilled and will run on RES with half tank or more of VP 110. It is my understanding that running any more than 10 pounds would be questionable, even with Carrillo rods and turbo pistons. 6-8 pounds with stock timing seems correct for a bike that will see limited street use and no track time? Hope to get on Dyno Tuesday or Wednesday.

[dhb]

In the late 70s and early 80s I ran 20 lb of boost in my 836 honda .I raced it every race and put 50,000 miles on my bottom end street driving with out  a failure.
The trouble I had was getting valve springs heavy enough to keep the valves from hitting the pistons  under high boost.I believe I ran fuel pressure as high as the needle and seat would hold.I also had to put a larger needle and seat in the carb and drill it out as far as possible then I had to flute the needle to get even more fuel in.After all this I could easily run 20 lbs.If you dont modify the carb you will never get the fuel you need to run high boost.

[dhb]

Forgot to mention I did have heavy duty rods and turbo pistons 8 to 1