I recently had my pump blueprinted and now the engine will only rev to 4200 rpm. The engine doesn't sound as if it is under load and the secondaries on the holley 850 cfm doesn't sound as if they are opening up. Do I need to change the spring on the vacuum secondary diaphragm spring?
My theory is that when I had the pump blue-printed that it changed the load characteristics on the motor and is causing the vacuum secondaries from opening up. I think I need to go with a lighter spring.
Has anyone else ever encountered this problem?
first, what was it spinning before?
Second, just how loose was the wear ring clearance that is now tight?
third, the only way to KNOW what the seccondaries are doing is to let someone else drive and you watch the carb.
fourth, if you can "feel" the seccondaries opening up, they are mis-timed. It should be a smooth and seamless change aas the engine winds up under load. They will not open up without a load if the engine is simply rev'd.
I have been battling the same problem! It falls flat on it's face after 4,000 rpm. I have changed base plate because throttle shafts were worn and installed extended jet's thinking it was starving itself and still works exactly the same.This past weekend I had the opportunity to watch carb while under full throttle and secondaries were not opening.When I opened them manually it was like a shot of nitrous and gained an easy 1000 rpm's.I am going to try some different springs in diaprahm and see if that works unless somebody has a better tip.
You can check the things that Smokin mentioned, but I'd take that vacuum secondary carb and sell it to some street car guy and buy a good 850 double pumper. There's not a jet boat alive that, in my opinion, benefits from a vac secondary carb. Fuel economy may be the exception on that.
While I agree with Garry on the mech vs. vac sec, have you checked the diaphragm itself? It might be torn or not sealing around the edges of the pot? One little leak will render it useless. As a test, you can disconnect the linkage for the vacuum pot, push on the little brass rod thereby compressing the diaphragm, hold your finger over the hole and release the rod. It should stay compressed. If it doesn't, it's not sealing.
If you stick with the vac sec carb and replace the diaphragm, it's one of the trickiest things on a Holley rebuild. The screws for the pot cover try to grab the rubber and spin it up which will tear it. I found that just a little grease on the screws helps to keep it from grabbing the rubber when you go to put the cover back on.
If nothing else that's the beauty of the double pumper carbs. No diaphragms to mess with.
I recently had my pump blueprinted and now the engine will only rev to 4200 rpm. The engine doesn't sound as if it is under load and the secondaries on the holley 850 cfm doesn't sound as if they are opening up. Do I need to change the spring on the vacuum secondary diaphragm spring?
My theory is that when I had the pump blue-printed that it changed the load characteristics on the motor and is causing the vacuum secondaries from opening up. I think I need to go with a lighter spring.
Has anyone else ever encountered this problem?
See if you can borrow a 800/850 DP to try out, you'll put that vac secondary up for sale.
As far as fuel mileage goes, depending on the amount of vaccum your motor pulls, you can have the secondaries cracked and not realize it. I like the DP because I know exactly how much throttle blade is open, not to mention the hit when you open er up.
After checking all the mentioned areas/springs etc. At 4200 you may not have enough vaccum to fully open the secondaries. What's the rest of the combo?
Gugs
Damn good tip on the diaphragm rerplacement with the grease. The way I check to see if I'm getting secondary oening is have someone driving the boat in a straight line at full throttle, then I check to see if they are opening or how much, by simply reaching back and trying to open the secondaries further. This will tell you if they are opening or how much thet are opening, but not why they may not be operating properly. And no, simply flashing the throttle open while on the trailer and out of the water will not open the secondaries unless you hold it wide open long enough to grenade the engine.
I just went and bought secondary diaprhragm spring kit and the instructions outline a few things but one that pertains to my situation is carburator restriction affecting vacumm signal.I went from having a spark arrester in an enclosed engine compartment to an open engine configuration with headers and snorkel type scoop that encloses the entire carb. Before I throw carb away I think the spring kit would be worth a try. my $.02 worth
I have a vac secondary on my boat and I like it, however after it was rebuilt, I get a loud, and I mean LOUD hissing sound around 3500 RPM's, and it doesnt go away unless its under or above 3400-3800, it will hiss all day if you keep it around those RPM's.... sorry to hijack the thread, but if anyone could help while we are on the topic I'd greatly appreciate it !!
What should I look at to see whats causing it, the operation of the carb by the way is flawless, all 4 open just fine and seamlessly.
I'm willing to bet the hiss is right at the opening point of the seccondaries. Open far enough and the engine volume as well as larger opening masks it. Reduce revs and it closes, and stops.
I also agree about a preference for a DP carb, I run a 4150 style, 700 CFM, and she runs hard as a lake-boat. Personally, unless you have a LOT of motor, a 750 would be much prefeable to an 850.
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