351c 2v Cam advice

[c9zx]

The cam is not that big. I think 14 in./hg. vacuum is good. 300-500 RPM fluctuations is not normal. What carb and distributor is installed? Either the distributor has a problem causing timing fluctuations or the carb needs adjustment/calibration or repair. Isolate one as known good and move to the other. Let us know what you find. Chuck

 

[tony-muscle]

Check timing to make sure it is not fluctuating in idle. I had a similar issue where my timing was fluctuating which affected idle speed. Just one of the possible causes to research.

Sent from my SAMSUNG-SM-G930A using Tapatalk

 

[ianpala]

The cam is not that big. I think 14 in./hg. vacuum is good. 300-500 RPM fluctuations is not normal. What carb and distributor is installed? Either the distributor has a problem causing timing fluctuations or the carb needs adjustment/calibration or repair. Isolate one as known good and move to the other. Let us know what you find. Chuck

I have the Summit 850306 ready to run distributor(4 years old),  a brand new QuickFuel slayer 600vs 4bbl carb and an msd flamethrower 2 coil(no idea how old). I had a loose coil wire for a while, don’t know if the intermittent connection could have damaged anything over time.  Any tips on a quick way to isolate one or the other as the culprit? Like I said before, it clears up significantly once the car is fully warm.

 

[TommyK]

You need to establish a good baseline from which to proceed with diagnosis/tuning. First I would remove the carb and verify/establish correct t-slot exposure (.020-040 showing below the throttle blade). Note how many turns out of the curb idle screw it takes to achieve this setting. Reinstall the carb and gently turn the idle mixture screws in until they seat and then back out 1 1/2 turns. Disconnect the vacuum advance and cap the port on the carb. Start the car and let it warm up until it will idle on its own even if it is surging. You may need to adjust the curb idle screw to get an acceptable idle rpm. Verify that the float levels are correct. Set initial timing at 10 degrees to start. Check for vacuum leaks. If no leaks I would turn the idle screws out another 1/2 turn ( 2 turns total) and see what effect if any that has on the surging. If no change turn the idle screws in so that they are 1 turn out total and observe effect. You can continue this process in a wider range of settings to see if a richer or leaner idle mixture has any effect on the surging. If any off these settings improve or alleviate the surging you have a direction to pursue with idle circuit tuning (lean or rich).

Once you have established what you think the best idle screw setting is you can repeat this general approach with initial timing settings to see if timing has any effect on the surging.

At first start up and during timing adjustment it may be necessary to adjust the curb idle screw to get an acceptable idle speed. If you end up turning the idle speed screw more than about a quarter turn in either direction your t slot exposure will no longer be optimized and that may require further sorting.

 

[c9zx]

+1 on TommyK's post. Check again for any vacuum leaks. I'd also check the float levels in the carb. If they are not right other adjustments are not effective. If the mixture screws are less than one or more than two turns CCW for best vacuum, the calibration will need to be adjusted. Less than one means the idle circuit is rich, more than two means the idle circuit is lean. With the "hotter" cam expect the idle speed to be higher than it was or the stock specification. Let us know how things progress. Chuck

 

[ianpala]

Thanks guys. Right now I had the mixture screws set at 1.25 turns out each. I did this using my vaccine gauge. I took them between 1 and 2 turns before and there was no impact, however I did all of this in park while the surging is less noticeable.

The idle screw is almost all the way out, barely had down to press down on the throttle bracket. My float levels are set at halfway up the viewer glass.

I have my vacuum advance coming from manifold vacuum not ported from carb. Should I change that? Also I did clamp the vacuum advance hose before to see if had any affect and it didn’t. Also clamped all other vacuum hoses, and sprayed carb cleaner around the manifold which made me feel there were no vacuum leaks. I will check again.

This Monday or Tuesday I will remove my carb and check my T slots to make sure they are kosher and also make sure my initial timing is right at 10 degrees to start before advancing (simply used my ear and vacuum gauge before). I will also re-do the carb settings initially while cold in drive when surging is at max. I just remembered having surging problems before the new cam, intake, heads and carb but I figured it was due to a flat cam lobe dropping the idle at that cylinder. The surging was not as severe which made me forget but it was definitely aparent here and there. I will do the discussed and get back. Thanks a ton.

 

[TommyK]

When the vacuum advance is connected to full manifold vacuum it can cause the type of surging you are experiencing if the vacuum threshold for operation of the vacuum diaphragm is exceeded. When this happens the timing fluctuates based on the varying vacuum level.

Without really knowing what you have for timing you are shooting in the dark. Before you remove the carb disconnect the vacuum advance and set the initial timing with a timing light and then test in gear. You will probably need to turn the curb idle screw in to get it to idle. If that solves the surging problem you should probably run the advance off a ported vacuum source. You can then proceed with your carb tuning. If you can get the car running decently I would suggest mapping the advance curve and based on that optimize the timing and then run through the carb again.

 

[ianpala]

Hey guys, went out of town for a little while but finally got back to the car and figured out the surging issue. It was a bad PCV valve. Should have figured it out, but it seemed to go in and out.

got it timed precisely too. 12 degrees initial, 35 degrees total. My vacuum gauge method had it set somewhere like 30degrees initial :/.

Ever since I've don't this work however, a new problem arose. from 1000-1500 RPM it runs like **** then its like a turbo kicks in and everything seems fine again. I'm thinking due to the new cam its running rich and I have to go down an inch or 2 in power valve size. My quick-fuel 600vs comes with a 6.5. Which isn't too far off but I'm thinking i need more like a 5. My vacuum gauge is acting up so I still haven't got a solid read but I thought my idle RPM is around 12-13 which should have been fine with that. Going to have to double check and also I'm going to drive around and get the read through those RPMS . There is also a chance i blew my diaphragm in it because I had a few carb back fires while dialing everything in. The problem just realllly seems like a power valve.

 

[c9zx]

If you have 13-14 in/hg vacuum at idle you do not need a power valve that opens at 5.0 in/hg. Chuck

 

[TommyK]

Can you be a little more specific as to the exact symptom? Runs like **** covers a lot of ground. If it is stumbling when you floor it the accelerator pump probably needs attention and/or the secondaries are opening too soon.

 

[ianpala]

So if I'm accelerating from a stop, or cruising real slowly, from 1000-1500 RPM it does what really seems to me is like a rich blurble and runs rough. (I'm going to pull some plugs to check color) Then at 1500 or shortly after it smooths out in one immediate moment with a boost of power. I'm not flooring it or anything, this is driving around the city. I'm getting awful gas milage, something like 7 mpg. It has also died while accelerating in this range when its cold, but only once. I will check the accelerator pump, and then the secondaries, never have adjusted secondaries. I'm going to replace my power valve regardless because I have had at least 3 to 4 carb backfires since installing, I know this can rupture the diaphragm and if I'm going to take it apart to check it i might as well put a new one in for $8. I will get one rated at half whatever my idle vacuum is.

Thanks!

 

[Don C]

The backfires indicate a lean condition. I would start with accelerator pump, although I would probably go through the entire carburetor, check all passages, jet sizes, etc. New does not mean correctly assembled/inspected. There might be debris in it from assembly or installation. The RPM range you're having problems with could indicate a problem in the transfer slots.

 

[ianpala]

The backfires were before I tuned the mixture screws and timed it, so I wouldn’t count that on the list of symptoms. And yeaaaa I've been thinking I oshould tear it down and put it back together to make sure, just been procrastinating since its my daily driver. Thanks

 

[TommyK]

The backfires were before I tuned the mixture screws and timed it, so I wouldn’t count that on the list of symptoms. And yeaaaa I've been thinking I oshould tear it down and put it back together to make sure, just been procrastinating since its my daily driver.  Thanks

That carb and basically all Holley style carbs made since the 90's have a backfire protection check valve so it is extremely unlikely the power valve diaphragm is ruptured.

 

[c9zx]

Most aftermarket carbs are shipped rich, it is safer than lean. Try dropping the primary jet size 2-4 numbers, readjust mixture screws and see if things improve. Chuck