'76 Spider bucking when I try to go over 55-60 MPH.

jprisbe · Post by **jprisbe** » Sun May 28, 2006 8:42 pm

Symptom:
She starts bucking if we try to push her over 55-60 mph. Never dies. If I back her off to less than 50 mph no problem. Ran fine previously when the weather had been cooler but now we have upper 80s and lower 90s. Started doing this yesterday and then it happened again today. Temp gauge shows normal operating temp.

Recent changes:
32 ADFA â€“ rebuilt over a month ago and has run fine since. I installed a Crane XR3000 with brand new Crane coil a couple weeks ago. Has run fine since that.

Details:
Yesterday we drove over 140 miles earlier in the day (before 4PM and the heat of the day) with no problem at all. Later in the day we pulled away from an intersection... I revved her kinda hard in 1st and 2nd and when I hit third she started to buck. I backed off and the bucking stopped. But did show up off and on the rest of the way home especially if I tried to get her over 60 MPH.

This afternoon (after 4 and in the heat of the day... again in the 80s and 90s) we drove about 10 miles and had dinner. Left there went a few blocks and stopped at the store. Left the store, got two or three miles down the road and she started bucking when I tried to get her over 55. Backed off to 45-50 and no problem. The last couple miles before home I intentionally tried to get her over 55 and she bucked, dropped back below 50 and no problem.

Related problem???
Last summer we had a similar problem on the way to FFO. In the heat of the afternoon, upon acceleration, she would buck and then die and would not restart. It would take several hours (I assume she was cooling down) and then she would start and I could drive several hundred more miles with no problem. I discovered my carb was loose on the manifold and after tightening the carb down the problem never resurface again. I checked the card today and it is tight.

Plan of attack??
I plan to test my fuel pump pressure, check for vacuum leaks at the carb and manifold, pull the fuel filter in the trunk and check for crud.

What else should I look at.

So Cal Mark · Post by **So Cal Mark** » Mon May 29, 2006 1:40 am

fuel pressure is a good place to start; typically low fuel pressure shows up in 3rd gear under a load. Along with a weak pump, restricted filters could cause the trouble. Make sure you check the screen in the inlet of the carb. And, cracked hoses between the tank and pump could cause the trouble too

Mad-Machine · Post by **Mad-Machine** » Mon May 29, 2006 12:52 pm

76 should be an electric fuel pump. If it is the original pump or an unknown age pump.. replace it with something new. The Facet electronic pump works well with a mostly stock engine if you can stand the light "tap, tap,tap" noise it makes while pumping the fuel. You can only hear it with the trunk open or before you start the car though.

Fuel filters are another good start. A clogged filter can let the bowl drain due to a lack of fuel flow. As Mark Stated, third gear is where you typically feel it.

Personally though, I am betting on the fuel pump. You symptoms mimic the ones I had years ago with my own 76. Sometimes she would buck and caugh and gasp for fuel when accelerating.. other times she ran perfectly.

jprisbe · Post by **jprisbe** » Mon May 29, 2006 7:34 pm

Thanks guys...

I going to start looking at things in the morning.

So Cal Mark · Post by **So Cal Mark** » Tue May 30, 2006 1:41 am

you could also have a corroded fuse box that is causing erratic powr to the pump

jprisbe · Post by **jprisbe** » Tue May 30, 2006 1:32 pm

Update.

The car originally had a mechanical pump but a PO installed an electric pump. It has an in-line fuse which is not corroded and the other fuses in my fuse box are all in good shape.

I pulled both fuel filters back flushed them and no crud came out at all. I checked fuel pressure with my vacuum/pressure tester and it showed 8 psi after the filters. This car has an inline pressure regulator just before the carb and itâ€™s set to 3 psi. I checked the pressure after the regulator and it seems to be accurate.

I started spraying carb cleaner (while the engine was idling) around the base of the carb and the vacuum line connections. All seemed ok.

Butâ€¦ when I sprayed carb cleaner along the top of the intake manifold where it connects to the engine, she revved up and then dropped back. I did this several times with the same result.

Iâ€™m thinking I need to tighten the intake manifold down. Should I go ahead and get a new gasket for the intake manifold and just try tightening it down?

So Cal Mark · Post by **So Cal Mark** » Tue May 30, 2006 2:33 pm

if you have an intake leak the car should idle very rough, since you're leaning out one or two cylinders. Try tightening the manifold first and retest with your carb spray. Changing that gasket is a chore

jprisbe · Post by **jprisbe** » Tue May 30, 2006 5:06 pm

I checked the bolts on the intake manifold and they were at 18 lbs.

I did tighten them just a bit but I still get the change in rpms when I spray the carb cleaner. It only happens when I spray in the location between the two bolts that secure the dipstick tube mount. No where else along the manifold/engine seal.

BTW, the car idles fine.

Maybe the vacuum port on top of the intake manifold between the card and the engine? I don't think there is anything else back there is there?
That port has two nipples. One has a 4" tube with a screw in the end (I think it went to the EGR valve at one time) and the other hose comes around to the drivers side of the carb base and connects to a port there.

Also, there is a brass nipple on the front right of the carb (next to the engine) and there is NO hose connected to it. Should there be?

So Cal Mark · Post by **So Cal Mark** » Tue May 30, 2006 7:33 pm

the thermo valve you're referring to was for the egr, but it screws into a water jacket, not vacuum. The nipple on the front of the carb had a hose to the vapor cannister, so it won't cause any problems if it's disconnected. If the car idles fine I doubt that you have a vacuum leak of any size

Mad-Machine · Post by **Mad-Machine** » Tue May 30, 2006 7:55 pm

I find this all pretty puzzling. Though, I do have to admit.. pressure and flow are two different things. I would replace both fuel filters and see what that brings you.

jprisbe · Post by **jprisbe** » Wed May 31, 2006 7:05 pm

Another update.

Decided to take her out late yesterday afternoon to see if problem persisted. Just before going I wiggled the carb to make sure it wasn't loose like last summer. I then tried tightening the carb down anyway. Each of the four nuts were loose enough that I could turn them at least one, to one and a half, full turns.

Took her out, got her to normal operating temp... pushed her to 4000-4500 RPMs before shifting any gear, pushed her to over 80 MPH, accelerated going up hill... basically tried to make her buck and she NEVER did.

I have lock washers under the carb nuts but I guess they didn't hold.

Could one turn of the carb nuts create enough air leak to cause bucking on acceleration?

jacksplace · Post by **jacksplace** » Wed May 31, 2006 10:46 pm

Absolutely (I think!)

glm8408 · Post by **glm8408** » Fri Jun 09, 2006 4:16 pm

I had a similar issue with my 78. It converted to electronic ignition with the electronic fuel pump. I'd be fine when it was "cold", but when it warmed up and I would push it up the hills here in Colorado I would experience the same response - bucking and eventually dying if I didn't back off. Once I backed off I could "limp" down the road for a bit and then it would allow me to get going again.

I fixed it by pulling the gas tank and discovering 24 years (most likely) of built up crud and rust. I took it to a shop and had them recoat the tank. I then spent a few days cleaning out the float and internal tank filter. That did the trick. I've not had any issues with fuel delivery since then. I've added two inline glass filters to the line to be able to keep on the gas. It's amazing how much gunk is in the fuel from gas stations - but, where else do you get the stuff?

Hope this helps.

Gary

jprisbe · Post by **jprisbe** » Fri Jun 09, 2006 4:43 pm

Gary,

Thanks for the info.

I haven't had another incident since the 31st of May... but then it hasn't been as hot either. Guess I'll see what happens next time it gets in the 90s here.

Jim DeShon · Post by **Jim DeShon** » Sun Jun 11, 2006 3:19 pm

My 77 did this once a few years back. The carb just worked it's way loose. If you still show a change in RPM when you spray around the manifold to head gasket, I'd still change out the gasket when I got the chance. It's not bad enough to keep it from running or to deteriorate the idle now, but there is trouble brewing there. ...J.D.