PMO injection quantity

[dipso]

I am getting a stumbling, hesitation problem when I floor it from idle.
It is O.K. once the RPMs are up and it doesn't happen in any gear except first.
Stock internal 2.7 with 40 PMOs.

I have read it might be a rich bog due to too much injection coming off of idle. I adjusted them with the syringe and they are about right.

I searched injection quantity and it seems like injection isn't even needed and only helps as the butterflies open. I have adjusted the nut down 1 or 2 turns and it seems a little better but I am not sure exactly how the internals of a carb work and don't want to adjust the quantity down too much. Can I do it too much? What would happen?
It won't affect the other mixtures will it?

Could I screw something up by adjusting too far? Or would it be safe to just close them completely and see if it helps.
I hear it would be hard to start without the injectors. Should I just turn the heck out of those things and then back them up if it hard to start?

Or should I just keep lowering the quantity one turn or two at a time.
Does it matter?

How exactly does the injection quantity affect performance?

Thanks.

[t951]

I have a 3,0L with PMOs. I have the same hesitancy. I was told that it was because I haven't had my distributor recurved.

Steve W is the guru with PMOs. Hopefully, he can chime in.

[James Brown]

PMO's are based on the flawed Weber design. They only have one accelerator pump per carb. The Zenith have 3 per carb. NO hesitation on acceleration!! Re curve or not, it won't help you at idle, you need some rpm's to advance the dissy. Sounds a little rich to me, try backing off the accelerator pumps a bit.
Just kidding on the PMO dig, love them!! And along with Steve, check with Richard Parr and Berry Hershon. There collective knowledge vastly surpasses the known universe of carburetion.

[Walt Fricke]

I had this bog coming out of slow corners with my Weber 46s on a 2.7 race motor. Reducing the acc pump output to almost nothing solved this.

With a cold engine I do have to pump the throttle quite a lot to get the engine to start. And sometimes to keep pumping as it starts catching until it is really running. I attribute that to the reduced acc pump output.

So I like your alternate plan of reducing output bit by bit until you have gotten rid of your hesitation. Though you won't hurt anything (other than temporary performance) by messing with this.

Mind you, my issues were full throttle bogging starting from 3,000 or 4,000 rpm, not from idle.

[snbush67]

I had to turn my accelerator pump nuts in about .5" to get the measure down to 6cc. It didn't make any difference.

More often then not the stumbling you describe is due to the linkage geometry not being perfect. It is a result of the cylinders getting different amounts of air and fuel which creates a boxing effect.

You have to check your air flow at idle and at 3000, this will tell you if the carbs are in synch.

Make sure the short arms are adjusted without any pre load from the linkage.

Check for synch at the pedal, this means someone has to be in the car, don't push on the linkage it isn't the same action.

Once you get it perfect you will know. It will be a silky smooth transition with no hesitation, stuttering, bumping, etc.

[dipso]

I'm pretty sure my side to side linkage balance is right and the carbs are perfectly in balance with the synchrometer.
I'm hoping it is the injection quantity. I will mess with that.
I can't screw anything up by lowering it , can I.

It runs great, it is just that I can't 'pedal to the metal' of idle. But, I want too.

[moneymanager]

I have a 2.7 with PMO 40's and have experimented a good deal with this. Like yours, my car stumbled badly when floored (but in second gear.) I first tried smaller float bowl control valves to increase (not a typo) the amount of fuel dumped on acceleration. This was wrong. I then reinstalled the original fbcvs and tightened the accelerator pump adjustment screws to reduce the amount of fuel added during acceleration. To sum up a lot of experimentation my motor runs very well with the jets/valves closed as tight as they will go. On mine, this amounts to about 13 full turns of the adjustment screws. If I were you I'd tighten 6/7 full turns on each side and see if it helps. If it does, add 3/4 more, etc. Keep track of the number of turns so you can keep the two sides the same and/or get back to where you started.
As I said, my motor runs well, but there is still a bit of a cough when the pedal if floored in second at low speed. I talked to Richard at PMO recently about trying a bigger fbcv to reduce (again, not a typo) the amount of fuel dumped a bit more. He steered me instead in another direction, sending me six very tiny plugs which will, when installed, block the topmost of four small holes in the progression circuit in each carb. He tells me I'll like this though I have yet to do it. This obviously has the effect of further reducing (by 25%) the amount of gas which will be dumped into the carb when you floor the pedal. I'm going to tackle this when I have time to pull the carbs off as it is a job that has to be done on a bench.

[dipso]

Excellent MM.
That was going to be my next question. I was going to ask if anybody knows how many turns it takes to reduce the quantity and by how much.

So by your estimation about 13 in is 0. So lets say injection quantity is 6 cc at 14 out and 0 at 14 in.
Going in about 7 would bring it to about 3cc.

That's exactly what I wanted to know.
Now one last question. Injection quantity is just off idle, until the butterflies open correct. So messing with the injection quantity is totally separate than any of the other adjustments off idle.

[moneymanager]

Yup, totally separate. The circuit simply supplies gas needed by the motor as you move off the idle circuits and get on the main circuit. You can so far as I know change this endlessly without affecting anything else.
Re the 13 turns, I doubt that this takes you to zero, or that 6/7 turns will take you to 3 cc. I think its more likely that fully tight takes you to 1-2 cc, some minimal amount. However I've never looked. But more important, you don't care. What you want is a motor that doesn't bog. If you start with both carbs dumping the same amount of extra gas, and you tighten each side ten turns and get good results, congratulate yourself. At that point you really don't care what the amount is.

[dipso]

Thanks.

[Shoepop]

Check your fuel pressure it needs to be 3.5 PSI, any more and you will get the bog you described.

[Walt Fricke]

I don't know about the PMOs, but the Weber acc pump system uses a sort of cam-like system, so I doubt that the change in quantity per squirt is linear with the number of turns on the adjuster.

Like Jim, I just played with mine until I got the results I wanted, and my nice graduated little vial sits unused in my carb jet box.

[dipso]

Originally Posted by Walt Fricke

I don't know about the PMOs, but the Weber acc pump system uses a sort of cam-like system, so I doubt that the change in quantity per squirt is linear with the number of turns on the adjuster.

Like Jim, I just played with mine until I got the results I wanted, and my nice graduated little vial sits unused in my carb jet box.

If you ever decide to use the vial, remember to notch it with a razor blade or knife before you use it. At the volume you are going for.

I didn't notch mine and lowered it in. Tested for volume. Pulled it out and the gasoline had washed all the ink off the outside of the syringe.
Duh.

Had to call Richard for another.

[moneymanager]

Just to add one point I should have made earlier. Reducing the injection quantity is said to make the car harder to start. I floor the pedal 6-8 times when cold and it lights instantly. Now that I think of it, I should experiment with fewer and see if I do as well.

[snbush67]

Originally Posted by moneymanager

Just to add one point I should have made earlier. Reducing the injection quantity is said to make the car harder to start. I floor the pedal 6-8 times when cold and it lights instantly. Now that I think of it, I should experiment with fewer and see if I do as well.

Cold for my car gets 5 pumps. Of course my conditions never change much here in Hawaii.

My injection is 6cc per throat.

[dipso]

My car starts with 2 full pumps. I don't even have to give it gas, it just starts and idles after 2 pumps.
I am sure the injection quantity is too high.

I think I'll lower it tonight. I don't mind giving it a few extra pumps in order to solve the bogging.
Hopefully.

[dipso]

Well, I marked the nut with a sharpie and started turning.
Started with 5 full turns, helped a little. I noticed it sounds better, probably unrelated.
Then went to 8 full turns, just about perfect.
Ended up with 10 turns and I think it is pretty good now.

Can go from idle to the floor as fast as I can push it and no bogging. Sure does whip out of a corner now.

Thanks for the help guys,
Lance

[RWebb]

congrats

you might as well post a pic to finish off the thread...

[dipso]

No pics of the sharpie marked nuts, but as you wish.



Just realized that was the wrong picture.
Here we go.

[dipso]

Wrong picture.
Here we go.