MicroMachinery Posted December 9, 2013 Report Share Posted December 9, 2013 I know a guy that just rebuilt his engine a couple thousand miles ago. He's got an issue with more vacuum being pulled through the rear SU carb than the front one. He broke it in with 38mm side drafts, but thought they were worn, as the front piston(cyl 1+2) at idle didn't rise as far as the rear piston(cyl 3+4). He switched over to a set of 46mm carbs and is having the same issue. When you put your hand over the front carb, the engine will run, albeit poorly. When you cover the rear carb, it feels like it wants to completely suck your hand in and it kills the engine right now. We've sprayed around the front carb, trying to find vacuum leaks, and none are obvious. The engine runs fine and makes plenty of power, it just seems like there's more vacuum being pulled from the rear 2 cylinders than the front. Has anybody heard of anything like this before? Quote Link to comment
ggzilla Posted December 9, 2013 Report Share Posted December 9, 2013 Yep. A compression issue can cause that. Give it a compression test. 1 Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 I was afraid of that. Quote Link to comment
ggzilla Posted December 9, 2013 Report Share Posted December 9, 2013 Don't fear - test it. Only takes 15 minutes and a $25 gauge. The compression may be fine. 1 Quote Link to comment
datzenmike Posted December 9, 2013 Report Share Posted December 9, 2013 The carbs need to be synced 1 Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 Yep. A compression issue can cause that. Give it a compression test. Thanks for the confirmation; we were going to do that today but didn't have a gauge handy with us. I'll snag one tomorrow from my shop. Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 The carbs need to be synced You've seen this before, Mike? Quote Link to comment
Just Joel Posted December 9, 2013 Report Share Posted December 9, 2013 Have you sync'ed the carbs? 1 Quote Link to comment
jvb5577 Posted December 9, 2013 Report Share Posted December 9, 2013 The carbs need to be synced Was going to say this. There is a screw on the side that allows the butterflys to open a certain amount. They need to be matched with a uni-syn 1 Quote Link to comment
datzenmike Posted December 9, 2013 Report Share Posted December 9, 2013 If the carbs are not synchronized then one is opened more than the other and doing more work. Basicaly the back two cylinders are doing all the work to idle the motor. Get a couple of feet of garden hose and adjust them by listening to the sound of the air being sucked in by holding the end to the throat of each carb. The Uni-Sync pictured above is very good for this but the hose will also get you close. 1 Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 I do have a uni-syn. When I put it over the front carb, it will go up to the first line. If I put it on the rear carb, it shoots to the top and kills the engine. The carb linkages and throttle plates appear to be in sync. The other day I held the pistons up while someone stepped on the accelerator. Both throttle plates appeared to open the same amount at the same time. If that's what you mean by "sync'ed", then yes, they are "sync'ed". Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 Pretty sure it's not because of the carbs.. carbs only respond to the vacuum. The engine is what creates it. I will consider all possibilities, however. Thank you. Quote Link to comment
Just Joel Posted December 9, 2013 Report Share Posted December 9, 2013 You need to adjust the throttle plates with the throttle adjustment screw so that the float in the uni-sync rises to the same position for both carbs. You need to do this both at idle and at high idle when the motor is revved over 2k rpm. It doesn't take much for them to be off and react exactly as you described. 2 Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 Thank you, sir. Quote Link to comment
wayno Posted December 9, 2013 Report Share Posted December 9, 2013 OK, see the screw above the yellow vacuum plug/cap, the brass throttle cable end is just touching it on my throttle cable bell-crank in the photo below, this and the one on the other side are how you adjust your carbs for idle, you can actually get it close enough by eye by adjusting them screws till both them pistons are open the same amount, but someone not familiar with these carbs should use a uni-sync. These are fairly simple carbs if serviceable, and I can tell you that within 10 minutes of starting to work on them. Looking down the throat of these carbs isn't going to help you much, adjust the carbs with the screws on the top close to the top housings and a uni-sync, and adjust the fuel mixture with the big nut below, that's about it, of course it helps to have the linkage working in unison also, meaning that both carbs move at the same time, and that is done with one screw on the back carb linkage on my SUs. 1 Quote Link to comment
dr.feltersnatch Posted December 9, 2013 Report Share Posted December 9, 2013 I agree they need to be synced then re adjusted 1 Quote Link to comment
Sealik Posted December 9, 2013 Report Share Posted December 9, 2013 Another thing to check is piston drop...<...that they are moving freely and at the same rate. Remove air cleaners and raise both pistons with left and right index finger...release...they should both drop at the same rate. If not...swap the tops of SUs (suction chambers). If not... remove tops again and then carefully remove pistons and springs Clean pistons etc...make sure springs are same length and reassemble. 3cc of ATF Try again...... 1 Quote Link to comment
Sealik Posted December 9, 2013 Report Share Posted December 9, 2013 Also...you're positive that the front (SU) throttle shaft bushing is not NFG? Typically people will drop that mixture screw to compensate for that vacuum leak...both screws should be 'unscrewed' the same amount if everything is copasetic 1 Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 Thanks a bunch, guys. The piston drop is equal enough on both. They both lift and drop very smoothly with the same "thunk". Quote Link to comment
MicroMachinery Posted December 9, 2013 Author Report Share Posted December 9, 2013 Also...you're positive that the front (SU) throttle shaft bushing is not NFG? Typically people will drop that mixture screw to compensate for that vacuum leak...both screws should be 'unscrewed' the same amount if everything is copasetic Well, I'm not absolutely positive, but I've sprayed around it, and it's not sucking anything in. I've been turning both screws in and out the same amount. Quote Link to comment
Sealik Posted December 9, 2013 Report Share Posted December 9, 2013 Spark plugs look all the same? I would give the engine a compression test...<....leak down test If that's good.......then proceed to Sus. 1 Quote Link to comment
exit64 Posted December 9, 2013 Report Share Posted December 9, 2013 Make sure you go over to 311s.org and read the techwiki article by Keith, the SU guru. The dogbone needs to be unhooked to really do it correctly too. Also Unisynch's suck, not precise enough. Get a synchrometer off of ebay. The air free flows through them and doesn't choke down the motor while you doing the adjusting. 1 Quote Link to comment
Laecaon Posted December 10, 2013 Report Share Posted December 10, 2013 Yea, I am not a fan of my Unisync. Cant get even readings, maybe with 2 though... 1 Quote Link to comment
MicroMachinery Posted December 10, 2013 Author Report Share Posted December 10, 2013 Nailed it. You guys weren't kidding; the smallest adjustment of that screw changed the vacuum drastically. We'll have to fine tune some more, but we have a good baseline now. Thanks again everyone! 4 Quote Link to comment
Zeusimo Posted December 10, 2013 Report Share Posted December 10, 2013 I Had This Problem On My Wagon And It Turned Out To Be The Ringlands On 2 And 3 You Sir Are Lucky Quote Link to comment
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