[Wojpi]

I have come into problem with Rpms. During hard acceleration let assume i start second gear 2000 rpm i put pedal to the metal rpms goes up to lets say 3500 and then they go back by around 200 rpms During this time engine is stumbling, after that they start to go up again very fast and very good the engine pulls very good.

Timing is set to 35 deg total advance, i am running speed demon 650, primary jets 70 secondary 78, squirter no 31, accelerator pump 30 cc, 0,5 inch spacer, mixture is set correct as i see the color of plugs coffe + milk. Engine is 318, with 360 heads , 2,02 valves and hughes engines camshaft 2328, performer RPM intake. KB dome pistons with compression 10,5:1, i am using 98 octane fuel.pover valve i 4.5, vacum at idling is around 14 inch

With all above and street tires, and 3.23 rear end the car was doing 14.2 quarter mile.

Does anybody have a clue how to solve this matter ?

[dave-r]

pover valve i 4.5, vacum at idling is around 14 inch

Power valve 4.5 inches? Try fitting something around 10 inches so you get some enrichment at WOT.

[Wojpi]

Originaly i had power valve that was marked 6.5 that comes originalyy with all Speed Demon carbs, i have fitted 4.5 so the spring is less strong. Do you suiggest to go with stronger spring ?

[dave-r]

The valve is there to enrich the fuel mixture if vacuum drops off such as when you open the throttles wide.

So the power valve needs to open a little below normal manifold vacuum.

I suggest you try fitting the valve that came with the carb back in. But I suspect you need an even higher rated one because nornally you fit a valve that opens just a couple of inches of pressure below your normal vacuum level.

Try the 6.5 and see if things improve. Then you will know if you are going in the right direction with the problem.

Another solution is to use a bigger pump shot. But this only gets rid of the problem on a drag race car as they only use two throttle positions. Closed and wide open.

[Wojpi]

Ok i have made a measurment at the idle it was like 10 inches of vacum, at full throttle it is like 5, ia have fitted 6.5 inch power valve and it is better now, i have also changed cam on the carburator to brown one so the accelerator poump is givin bigger shot of fuel. Now i am going to work on ignition. I will change teh springs to two light silver what should normaly change the ignition curve, so i will have total advance at 2000 rpms. Do you have any experience working with this curve ? For me it is all new as i have no experience with engines.

[dave-r]

Ok i have made a measurment at the idle it was like 10 inches of vacum, at full throttle it is like 5, ia have fitted 6.5 inch power valve and it is better now

I would have fitted a 7.5 valve with that much vacuum at idle. You would need less pump shot too.

i have also changed cam on the carburator to brown one so the accelerator poump is givin bigger shot of fuel.

What size pump shot nozzels are you using? It normally better to increase the size of the nozzles first rather than the cam.

Now i am going to work on ignition. I will change teh springs to two light silver what should normaly change the ignition curve, so i will have total advance at 2000 rpms.

You want about 18 to 20 degrees initial and 15 to 17 degrees advance in the distributor. Giving you a total of around 35 degrees at 3000rpm. You only want about half the distributor advance in by 2000rpm. This normally means limiting the amount of advance in the distributor.

[Wojpi]

Sorry I am green, do you mean squirter ? I have squirter no 31, accelerator pump is standrd size that comes with demon carb, before i had pink cam, now i have brown which is almost the biggest.

"You would need less pump shot too." So you think i should go to back to pink cam ?


Correct me if i am wrong, initial timing you set up by turning your distributor, advance you set by changing the bushing in tha distributor.

So i should set up 17-20 initial (at lets say 800 rpm, idling) + bushing in the distributor limiting total advance to 35. Then i should choose the springs to have around 25 degrees at 2000 rpms. Is my thinking correct ?

[dave-r]

Sorry I am green, do you mean squirter ?

Yes.

"You would need less pump shot too." So you think i should go to back to pink cam ?

The power valve and the pump shot both do the same kind of job. If you fit the correct power valve you MIGHT need a smaller pump shot than before.

Changing the cam on the pump shot changes the duration of the shot. NOT the amount of fuel.

The amount of fuel is determined by the size of the pump. Usually 30cc or 50cc.
The length of the shot is determined by the cam.
How fast the fuel goes in is determined by the squirter size.

Fit the correct size power valve and go back to the cam you had before. Then, if you still have a bog when you open the throttle fast, increase the size of the squirters until it just goes away.

Correct me if i am wrong, initial timing you set up by turning your distributor, advance you set by changing the bushing in tha distributor.

Correct.

So i should set up 17-20 initial (at lets say 800 rpm, idling) + bushing in the distributor limiting total advance to 35. Then i should choose the springs to have around 25 degrees at 2000 rpms. Is my thinking correct ?

Example:

18 degrees initial.
18 degrees in distributor.

Total = 36 degrees.

Half distributor degrees = 9 degrees.

Distributor degrees at 2000rpm = 9 degrees

Total degrees at 2000rpm = 9 degrees + 18 degrees (initial) = 27 degrees.

So I would be looking for about 27 degrees total at 2000 and 36 total at 3000rpm with no advance past that speed.

[dave-r]

But don't forget. The actual degrees you run depends on your cylinder pressure and fuel quality.

[Wojpi]

Everything looked to be Ok until today when i have discovered that half of my spark plugs are black and other half is white color. It seems like half is rich ond the other half is lean. Cylinder 1 7 4 6 are rich (black spark plugs) cylinder 3 5 2 8 are rather white so lean. Does anybody have an idea of reason of that. I have edelbrocke performer RPM manifold so it seem that left side of manifold is feedeing 1 7 4 6, carb is Speed Demon 650 jest are 70 primary and 78 secondary, all idle mixture screws were adjusted tha same what was one full turn from fully seated.

Any suggestions will be highly appreciated, unfortunatelly ther is nobody here i could ask, and my knowledge is very limited.

[Eddie]

It almost sounds like a main jet/passage has a restriction. Was the carb new before you used it?

[Jon]

Possibly a vacuum leak on the one side. Do you have power brakes? The booster line is usually connected near the #7 runner. Just pinch the line with pliers and see if the idle changes. Another possibility is a blockage in the jets and/or other circuits on that same side. Are all of the unused carb vacuum lines capped off? I'm sure others will also have ideas. Good luck.

[Wojpi]

Yesterday i disasemled the carb and used a lot of compressed air in all the circuits, we will see if sth change, i have alo another demon that is 750 so aftre installing smaller jest i can alo try it.
What i also did i have replaced the no 70 jets with 71 and performance of the car has improved, 78 i have left unchanged.

I do have power brakes so i am going to test the vacum line. Brake boster is new so i hope there is no leak there. I am using 0,5 inch spacer on the carb do you think there can be some leak too ? I have done it with two gaskets.

[Wojpi]

Problem solved. I have changed carburator to speed demon 750, just reduced jets size little bit, accelartor cam is pink,PV is 6,5, no hesitation anymore Engine runs smooth.

[dave-r]