[Project'68]

Another learning moment along the way...fuel pump pushrods! Turns out not all cams are made of the same material. Im guessing everyone here already knows this or doesnt run a silly mechanical fuel pump anyway.

My last roller cam I had a chromoly pushrod in there, and it ground the pump lobe down. You can see the end of the pushrod is gross looking.

So the solve is a bronze tipped pushrod. Well....none in stock ANYWHERE. So I bought one for an oldsmobile block which means the pushrod is .200 longer and I shortened it.

Its working for now, and ill put the correct one on backorder.

 

[Project'68]

Ive been out flogging the Nova a bit, and things are generally holding together. AFR looks rich all around which is better than the alternative. I haven't done any WOT pulls yet as I want some time for the cam to break in. Oil leaks to the combustion chamber seemed fine initially but now the plugs are a bit wet again. I just re-torqued the intake manifold which hopefully will help.

I also have a really weird issue in 2nd gear at 3200rpm under medium load. Its reproduceable and the car vibrates pretty hard and loses power immediately. It feels like something major is resonating (maybe the driveshaft?) or like the ignition totally gives up. I can load it pretty hard in other gears and not see this issue, so Im not sure whats going on yet.

I have some new parts sitting in the garage so ill update more when I start the install.

 

[kimmer71]

Sounds like a certain RPM in the driveline is bad mojo.

 

[Project'68]

Sounds like a certain RPM in the driveline is bad mojo.

Maybe...but 3rd gear pulling hard I can run it through that RPM without issue. Maybe its a combo of driveshaft and rear end speed? If thats the case it should be solved soon ;-)

 

[kimmer71]

Maybe...but 3rd gear pulling hard I can run it through that RPM without issue. Maybe its a combo of driveshaft and rear end speed? If thats the case it should be solved soon ;-)

Since it's only that specific rpm in 2nd, it's that particular rpm on the driveshaft possibly. Any other gear/engine rpm combination is a different drive shaft rpm. If you shift into 3rd when it starts and let the engine rpm drop to maintain same vehicle speed, is the vibe still there? If you push in the clutch, is it still there? Could it be a wheel/tire balance issue? you might try to recreate it on jack stands unless it's so violent it could shake the car off the stands. It seems like a drive-shaft issue based on what you are describing. Any reason to suspect it?

 

[Project'68]

Since it's only that specific rpm in 2nd, it's that particular rpm on the driveshaft possibly. Any other gear/engine rpm combination is a different drive shaft rpm. If you shift into 3rd when it starts and let the engine rpm drop to maintain same vehicle speed, is the vibe still there? If you push in the clutch, is it still there? Could it be a wheel/tire balance issue? you might try to recreate it on jack stands unless it's so violent it could shake the car off the stands. It seems like a drive-shaft issue based on what you are describing. Any reason to suspect it?

Yeah Im really not sure. If it put it in 3rd when it starts to vibe it stops shaking and everything is fine. The driveshaft is newly rebuilt (with a larger tube for the center section) and has new yokes. It could be that under enough load the rear end rolls upwards and the driveline angle goes flat and allows it to shake.

Once I fix my fuel tank issue Ill be regearing the rear end with some 4.10s which should be much nicer than the silly 2.56 which is in there now.

 

[kimmer71]

I didn't think of driveline angle but it makes sense if it under hard loading.

 

[Nova Thug]

Seems to be isolated to 2nd gear based on what you have mentioned here. A driveshaft is either balanced or it’s not.. If it’s not balanced you will know it.. Maybe something in the transmission 2nd gear. I’m wondering if harmonics could be playing a part in this. Some confluence exhaust drone, engine rpm, and maybe a dead spot in the throttle response. I imagine that in 2nd gear at 3200 rpm’s with 2.76 gears you are moving pretty quick.. Could you be having some chatter from axle wrap being like right on the edge of the rpm range where it wants to deflect the leaf springs..?

Just some thoughts here..

 

[Project'68]

Thanks for the responses. Unfortunately, Ive been leading you guys down the wrong path due to me not testing enough 🙃 @Nova Thug you are right about the driveshaft. If it was out of balance it would be a clear issue.

Today I got a nice long cruise to work and back on the freeway and got to play around a bit. I wasnt paying attention to RPM this time, but was monitoring AFR. I found that in any gear, the deeper I get in the throttle the richer everything gets until it just wont run any more. AFR in cruise is about 11.5 which is already super rich, and well before WOT its in the 9s. I had messed around with the choke because I cant get a good cold start idle. When I got home, I left the car running (so it was fully hot) and pulled the air cleaner. The choke blade was a bit more than half way open with the thing fully hot. I think this is likely the culprit. Im dialing the choke way off and will just manually keep the idle high in the morning to see how it behaves on the freeway.

I also really want to get it re-geared, but its going to be a while before I have the time to tear it apart.