Carbs are poping and hesitating

toddu · 05-08-2014, 03:23 PM

So, still getting acquainted with my PMOs. Car idles great, but on accel and highway cruising it hesitates and pops.

I cleaned out fuel filter and idle valves, but no better.

What I did notice is the FP regulator is a solid 3.5 at idle. If I give throttle at the carbs, the FP gauge bounces all over, from about less than 2 to over 4. I'm sure this can't be correct and is probably my issue (or at least not helping).

I have an inline filter prior to the regulator, but it's mediocre. Thoughts? Could my filter be restricting? Anything else to check? Haven't been able to enjoy my new motor yet :-(.

Todd

Targa Time · Targa Time's Garage

Being no one else is replying I'll give my 2 cents worth.

The carb set-up from PMO is just a starting point and usually needs adjustment.

You may be running lean on the mains. How do the plugs look? Consider a AFR at WOT to be safe.

Did you clean the carbs before installing them? Although sealed in plastic mine had manufacture contamination that was plugging things up until removed and cleaned.

As far as the FP reg. goes. Mine does the same thing. Don't know why, but it does not seem to make a difference in performance.

If you post your engine build and current carb set up someone may be able to make some suggestions.

toddu · 05-09-2014, 03:51 AM

Engine was on a dyno and completely tuned\setup. So the gauge behavior is normal then?

Todd

toddu · 05-09-2014, 05:27 AM

And too add, I did drive the car about a month ago on its maiden voyage and didn't experience and popping, etc. Was chasing an oil leak at front cooler, so only drove it about ten minutes. But now its behavior is basically right out of the driveway.

Todd

Targa Time · Targa Time's Garage

From what I can tell the gauge just acts that way. I currently have a broken face and will be replacing it with a upgraded gauge. Will be interesting to see if that makes a difference.

Your tag says that you are in NC. Has there been much of a temp change sense the initial dyno set-up?

Did the dyno tune set up the accelerator pumps? If the popping is just when you put your foot into it the accelerator pumps may not be shooting enough volume, causing a momentary lean condition and popping.

Just a thought.

toddu · 05-09-2014, 07:00 AM

It's definitely warmer and more humid then when I first drove it, but the engine was dyno'd in FLA. I notice the hesitation more when I mash the throttle, but it will sputter and pop with a steady foot. Can't tell if it's through carbs or exhaust, kind of sounds it's both.

Todd

rsscotty · 05-09-2014, 07:09 AM

What are the details on your ignition system....CD brand, distributor, wires, plug heat range etc.?

toddu · 05-09-2014, 07:17 AM

MSD, twin boxes and coils (car is dual plugged). Engine was dyno'd with my boxes and coils. JB twin plug dizzy. Would have to look up plugs, but think they are what Richard recommends for PMOs. I need to pull a few and see how they look. Need to check float level at higher RPM as well.

Targa Time · Targa Time's Garage

The idle jets on PMO's tend to have problems with the idle circuit jet getting obstructed, causing symptoms like you describe.

Try pulling them and blowing them out with compressed air and see if it helps.

Being your engine was Dyno tuned I doubt that the minor change in temp. has anything to do with it.

Targa Time · Targa Time's Garage

Sorry just reread your first post. You did that already.

toddu · 05-09-2014, 07:37 AM

Yes, was all excited to take it out after and enjoy, but no love :-(

Spent a ton of money and between the oil leak (crappy AN fitting) and now this I haven't been able to really check it out.

Have a really good pcar shop 5 minutes away, might resort to having them take a peak. I have very limited time.

Thanks for all the ideas so far!

Todd

LJ851 · 05-09-2014, 09:55 AM

Remember the first rule of carburetor problems: It's not the carburetor.

How do your plugs look? You could easily have a fouled plug(s) causing the problem or another ignition problem. What are the air/fuel ratios during normal driving?

G50911 · 05-09-2014, 10:03 AM

what size are your idle jets? i had a similar problem and i upped it to 65's on my 2.7 and it cured it 100%.

toddu · 05-09-2014, 11:34 AM

I'll pull the plugs tomorrow. Put a timing light on and verified all 12 were firing. Also verified the piping seemed to be through the exhaust. More of a spit then a pop. More tomorrow.

Todd

toddu · 05-09-2014, 11:36 AM

Quote:

Originally Posted by G50911

what size are your idle jets? i had a similar problem and i upped it to 65's on my 2.7 and it cured it 100%.

60's

G50911 · 05-09-2014, 12:07 PM

if you're spitting i'm gonna bet you're too lean. do you have any leaks? how old are the carbs? have they been rebuilt? i hear the throttle shafts can wear down it's seat and you get leaks that way. any chance your accelerator pump is messed up?

i'd still try upping the idles to 65 and see what happens.

1QuickS · 05-09-2014, 12:58 PM

Did you buy PMOs from a PMO dealer? Were they configured for your engine as it is currently constructed?

Private Mail sent.

snbush67 · snbush67's Garage

I was able to eliminate similiar symptoms by raising my fuel pressure from 3.5 to 4lbs. It seems the difference between a full tank of gas and an 1/8th of a tank results in about a 1/2 lb of pressure difference. I have found that by adjusting the pressure at idle with a 1/2 tank of gas to 4lbs works best.

toddu · 05-09-2014, 05:52 PM

Quote:

Originally Posted by 1QuickS

Did you buy PMOs from a PMO dealer? Were they configured for your engine as it is currently constructed?

Private Mail sent.

I bought the carbs slightly used and have purchased several related items from Richard and talked with him several times.

The carbs were sent to the builder and pretty much everything about them was changed (jets, emulsion tubes, venturis, etc), all based on the dyno pulls. Should clarify that the entire engine was rebuilt as well.

I did remove 2 bottom plugs from opposite banks. Looks maybe more on the rich side than the lean side, but not fouled I don't think. I'll look at more to rule out a difference in cylinders.

fred cook · fred cook's Garage

Quote:

Originally Posted by toddu

It's definitely warmer and more humid then when I first drove it, but the engine was dyno'd in FLA. I notice the hesitation more when I mash the throttle, but it will sputter and pop with a steady foot. Can't tell if it's through carbs or exhaust, kind of sounds it's both.

Todd

As a general rule, popping and sputtering thru the carbs indicates a lean condition and popping in the exhaust a rich condition. The best way to tell is to install an AFR gauge (at least temporarily) and get some readings. A lean condition could be caused by any of several things. Have the float levels been checked? Have you pulled the emulsion tubes and checked for debris at the main jet? Any debris or trash at the fuel pump intake? Good luck with your search!