I'm working on fine tuning my freshly broken in 327. I hooked up my vacuum gauge to full manifold vacuum and it seems to be reading backwards. At idle, it shows 10" hg and when you give it some gas the vacuum rises to 25"ish. Weird. Can anyone help explain this or let me know if I'm doing something wrong? I took a video of the gauge but can't seem to upload it. Thanks Posted traditionally using tin cans and a string.
Sounds like it is connected to ported vacuum, not manifold. The needle should be reading the opposite. Check a different port on the manifold or carb. -dd Posted using the Full Custom H.A.M.B. App!
It's a nipple directly connected to the intake runner Here's the gauge I'm using. Thanks guys Posted traditionally using tin cans and a string.
Do you have a fairly big cam? If so, it could explain the lower idle vacuum and then why it increases with rpm. The big cam has too much overlap at low speeds and causes the low vacuum. Once speed increases the overlap is beneficial and the engine "works" better, and vacuum increases.
My cam is pretty moderate. It the comp xs268s the specs are: 230/236 @ 0.050" 0.016" valve lash (solid lifter) 0.490/0.501 lift 110 degrees LSA Installed straight up and degreed Posted traditionally using tin cans and a string.
I will. I removed the distributor to replace it. The old one had some slip in it. I will try tomorrow. The secondary carbs seem to seal well though. Posted traditionally using tin cans and a string.
Gimpy, you just got me thinking. I don't think in even opening the secondary carbs. Do you think that perhaps the engine is demanding more air than the center carb can supply at pet throttle thus creating a higher vacuum scenario? Thanks for the advice so far Posted traditionally using tin cans and a string.
It might just be me, but non the less; If you put a gauge on where you have it now, you Just get vacum, created by the injector effect from a pipe injected a cross an airstream. This is complet useless for any purpose, except "draining" a vacuum booster, or creating an vacuum driven RPM gauge!! It only tells you that there is air flow in your manifold! If you wish to balance you carbs or use is a diagnostic tool, you must connect your gauge just by the base of the carb, even better in the carb by the butterfly(s). If you wants to balance your carbs to each other, you will need one gauge Per carb. And a outlet Per carb. If you want to use it as a economy meter one is enough. Try Youtube, there is some great on doing honda cb400/4. I know they are OT and build by the "enemy". But you might find an American made video this ;-)
Anyone know how to post a video from the phone? I can show exactly what I'm talking about. Posted traditionally using tin cans and a string.
That's what I thought. This is really weird. I readjusted my valves last night too to see if they had anything to do with it. They were a little loose. Also, re-verified TDC. I will verify timing after the neighbors are up. Posted traditionally using tin cans and a string.
It's funny you mention that; I purchased a new gauge yesterday because i thought there may have been an issue with my old one. Same results on either gauge. Posted traditionally using tin cans and a string.
If I'm in fact is wrong, then please prove me wrong, by doing it the way I learned it. Not to be dis-respectful in any way, but still this is what I have learned as the correct way! What is it you want to do? Balance multiple carbs to each other? Because I get a different reading from the base of my Weber, then from the pleanum. And in my training I've learned that the vacuum in the runner only tells me how well the individual sucks/performs.
Bernoulli's effect can throw vacuum readings off if the port opens to an area of high flow velocity. I would try a different port to read the vacuum & use the new guage too. With both connected you can compare readings & see if there's a discrepancy. Over carburation can mean low vacuum and stumbling at off-idle rpms. (Just above an idle, at which opening most cruising takes place.) But at idle speed, this isn't normally an issue IMO. If you have a vacuum leak, and the idle mix was raised to mask the leak & allow an idle, then the engine will have low vacuum at idle. It will normally stumble too, unless you're set up very rich, at which point it won't wind up well. The effect of vacuum leaks is normally worst at idle to low RPM. Huge airflow at high RPM makes a small leak ineffective then.
My carburetor seems to be pretty much on point. No stumbling and the plugs are a nice tanish white. I was using the vac gauge to help with timing and diagnose a small miss. Thanks Posted traditionally using tin cans and a string.
I don't need to balance the carbs as they are run progressively. The outer carb throttle plates are pretty well sealed and not leaking when running on the center carb. My original question was to see why the vacuum gauge seems to run backwards (increasing in vacuum as the throttle is open rather than losing vacuum). I appreciate your insight Posted traditionally using tin cans and a string.
No change with the end carb linkage disconnected. I got it running pretty well though. It's a bit lean and has some surging when I romp on it. Getting closer! Posted traditionally using tin cans and a string.
The question of the vacuum in the runner is not really the same as a multi-carb motorcycle with individual runners - those do have to be synced and can have different vacuum based on each carb opening. The manifold on his car is all (relatively) constant vacuum. Yes, at the base of the carb or right in the plenum is better, but total vacuum should be about the same. The runner just makes for a bit more needle shake.
Yes, totally different scenarios when you have a common plenum vs one carb per cylinder as the multi-carbed motorcycle that was referred to in an earlier post.
I have lakes headers that are baffled with a half a glass pack in each one. This definitely mellowed the noise to an acceptable neighborhood cruise level. Do you think that these are causing excessive back pressure and associated drama? Thanks Posted traditionally using tin cans and a string.
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