30 runs great at idle- throttle up different story.

[danrossi]

We have a 30 that we are having trouble with its sputtering and backfiring at half to full throttle. we eliminated the tank by running an aux. fuel tank. changed carbs.(have 3)installed a brand new distributor.modern which we were running before. Retimed and set points. getting spark to all cylinders. Any suggestions? also the new distributor came with new points and condensor..

[Mitch//pa]

Try a coil might be breaking down under load.. can also put an in line spark tester to cap and look for int or weak spark

[ScottNW/Ohio]

Sounds like an air/fuel mixture problem - intake manifold gasket leak ?
Would have suspected GAV mixture problem, but not likely on all 3 carburators

[danrossi]

Thanks fellas, do you think it could be a valve problem? car not run that often, only in the summer in NJ.

[Mitch//pa]

could be alot of things even an exhaust pipe restriction from varmits

coil wired backwards

am shying away from manifold leak because it runs good at idle up to 1/2 throttle

[1931 flamingo]

Sponsored Links (Register now to hide all advertisements)

Certainly won't hurt to add 4-5 ozs of MMO to your aux tank. How about a compression ck??
Paul in CT

[Tom Wesenberg]

A vacuum guage can tell you a lot.

[James Rogers]

Had one like that. Had a bad plug breaking down under load. Try a new set.

[RichPA]

New Motorcraft plugs cured mine.

[TinCup]

Sounds like you have done a lot to narrow it down. I like Mitch's suggestion to swap coils and see what that gets you. You can also use the wd-40 test and spray it around the intake to see if it affects how it runs. This would indicate a manifold leak. You say it runs great at idle which in my book would eliminate valve timinig but would be worth checking.

[danrossi]

Tom, where do we put the vacuum gauge?

[danrossi]

did try swapping coil wires, got worse.lol ears still ringing ....

[danrossi]

running champion w18's good plug?

[James Rogers]

Originally Posted by danrossi

running champion w18's good plug?

18's are good plugs but I like the W16Y better. Both champion. I would just about bet it is a bad plug.

[Tom Wesenberg]

Originally Posted by danrossi

Tom, where do we put the vacuum gauge?

About mid 29 and later intake manifolds should have a vacuum port for the vacuum wipers. You can connect a vacuum guage to that port.

[700rpm]

How old is the gas? And even tho you have anew condenser in there, you might try another. Also check your GAV for 1/2 open while cold, 1/4 open when warm.

[danrossi]

what should the readings be on the vacuum gauge?

[Tom Wesenberg]

Originally Posted by danrossi

what should the readings be on the vacuum gauge?

A good running engine should show about 18 to 22 inches of vacuum. My 28 doesn't have a vacuum port, so I can't easily connect my guage. What you are looking for is a steady reading. If it's steady at idle, but starts to flick at higher speeds, that would indicate a sticky valve not closing as it should.

If it's steady and near normal at idle, but drops off while staying steady at higher speed, that could indicate a restricked exhaust.

[Napa Skip]

A very informative (to me) link concerning engine diagnosis using a vacuum gage was provided in reply 21, posted by HoarseWhisperer to the 4/19/12 “Why does my engine die when stopping?” thread. The link is:

http://www.classictruckshop.com/club...ts/vac/uum.htm.

[rglaab]

Thanks for the chart, That's a great tool to help us all

[danrossi]

Sorry for the late reply, just to lyk. It was the coil! running great again! thanks for all the replies.