Any Carburetor Tuning Tips for a first timer?

[stancalame]

I have a 1991 OMC 547APERGD WITH CONE CLUTCH KING COBRA OUTDRIVE and this is all related to another thread I have regarding difficulty not shifting out of forward into neutral. I am working to insure my idle speed is in the 450-500 RPM range as sepcified. I'm preparing to turn my carburetor and check engine RPM. Bought an "apparent" gently used Sears dwell/ tach tachometer from a local guy on Craigslist and purchased new vacuum/pressure gauge for tuning carb properly.

I know I need to connect vacuum gauge to full intake vacuum and I hope it's obvious where I can connect or tee into a line. Once engine is warmed, can I disconnect or tee into the vacuum line that is on bottom of carb that is connected to the choke diaphragm and get full vacuum? See photo.

Carb is Holley 650, 4175 spreadbore. New kit in it 2 years.

If not there, then where?

 

[alldodge]

Yes, you should be able to do that. If you Tee then the gauge can be left on while running higher rpms, but for tuning, just connect it straight

 

[stancalame]

Thanks.
Since you mentioned it, should I monitor the Total Advance timing values referenced in my manual? My primary goal is to understand if my boat tach is accurate and eliminate RPM as an issue for the not shifting into Neutral out of Froward. So I intend to achive the idle RPM and then test the shifting.

I've spent a bunch of time researching timing and it seems that I should really be focused on appropriate total ignition sparke advance at max RPM. I'm puzzled because the manual lists total timing in a table, but only references Idle RPM (in gear) in the section on Finish Timing.

If I think I can trust the test tach, then should I adjust the distributor position to meet published at WOT RPM (whatever that turns out to be?) ​Currently I can only achieve ~3500 RPM but I guess I have a separate issue. Any Comments on that issue.

The manual indicates 30.4 degrees @ 4600 RPM. Do I need to beconcenred with that? For example, at current state I can only get about 3500 on boat tach at WOT. Using test Tach, if dont get the same timing as the manual at higher RPM, then do I adjust for value at higher RPM or stick with Idle Timing? I assume if I adjust for advanced timing then it will change my idle timing.

I'm probably over complicating but would appreciate your perspective.

 

[alldodge]

Your getting into a bunch of other questions and not just carb tuning. For the most part carb tuning is for idle to around 1200 rpm's. After the idle circuit is done the motor should idle correctly in or out of gear.

Total timing deals with the distributor and if it can provide the total amount of timing advance, so this is another issue. We can get into it, but first lets get the idle and initial timing correctly set.

If your hand helm tach equals the boats tach then your good and can use it.

WOT rpm is another issue. If unable to achieve this you never readjust timing to try and achieve. Adjusting timing can put the motor in a lean and/or to far advanced timing and can and will damage the motor.

First make sure timing is set correctly, then adjust carb to achieve the in and out of gear rpm and functionality.

 

[stancalame]

Sounds resonable. Thanks fot the help. I do tend to get more than one ball in the air. So I'll just focus on tach confirmation and shifting. Thanks for the pacing.

 

[Scott Danforth]

no need to plug a vacuum gauge in to tune it. you read the plugs

new kit 2 years ago doesnt mean squat if you have crud in your fuel system. you could have a blocked idle circuit or a ruptured power valve

make sure your floats are set right.

if your motor backfired, and the carb is prior to 1991, then most likely the power valve is ruptured. Holley didnt start installing the anti-blow-back check valves until 1991. if the power valve has ruptured it will run really rich, which you may be able to adjust the idle mix enough to get it to idle, however it will fall on its face coming off idle.

my suggestion, get a new power valve, and a pair of metering plate gaskets and bowl gaskets. get the blue ones because they are reusable. if you have the blue ones now, just pull the bowls off and look.

not shifting into neutral on a cone clutch drive means your low on gear oil or your shift cable is not adjusted right, or both.

post your carb numbers and I can tell you what should be in the carb for jets and a power valve. however for a stock 5.7 liter with a holley vac-secondary, my first guess is you should be about a #70 primary jet and about a #94 secondary jet with a #35 or #25 power valve. on my warmed over 5.7 I was running #78 primaries and #98 secondaries.

 

[stancalame]

Wow lots of info. I'm already to pack up for the lak and tube away. My carb is list number 80390. Model 650, 4175. In the kit I put in 2 years ago, I'm pretty sure it can with power valve.

Last time I adjusted timing, it eliminated the backfire. What is best way to tell if it's running rich? Is that what you
Meant by read the plugs? Is it cost effective to only but the gaskets and power valve? Seems local airports stores only sell the kits. Last time I used a Renew kit # 703-34 but I don't recall changing any jets. Don't they came with the kit.

 

[stancalame]

Sorry for typos. Working Mobile. Here is what I meant to say. Headed to lake to tune, power valve came with kit, so what blows power valve? Too much vacuum?

I want to put gauge in it to see what I see. I just serviced lower unit including gear oil. Before I tear into the shift cable I wanted to eliminate any rpm too high issue. Plus when I tear into the control box, I'm anticipating g trouble getting shifter off. I posted a while back that it wo t come off. I'll take a video today and post. I need to keep boat together until after 4th then I can get into it.

Thank you for your insights. My gut feel is the shifting is a cable issue. However my idle has never been in the 540-650 range so I want to test the tach.

 

[Maclin]

A backfire can rupture the Holley power valve diaphragm, only takes one. After that fuel dumps straight into the plenum from the carb bowl.

 

[alldodge]

Yes, reading the plugs means your looking for anything other then a light tan or brown color on the plugs. The darker it gets the more fuel your running, the whiter it gets the leaner your running.

Jets don't come with the kit but normally they are not replaced unless something tells you they need to be

 

[stancalame]

Hmm, interesting. I pulled plugs a couple of weeks back to look at them and recap. Some were definitely darker than others. My engine idols well and seems to run well out of the hole and through all RPM ranges. Since I adjusted the timing I haven't had any more backfires I assume even with blown power valve engine would run seemingly normal?
Given that in the past I've run 87 octane without fuel stabilizer, the carb likely need another kit.

I know the last shitheads blue gaskets in some part of it but I don't remember if it was on the fuel bowl. Should I order a re-new kit and double check? I checked my vacuum and it was running 15 inches of mercury plus or minus one, it was a little jumpy at Idol but steady and higher RPM

While tuning carb yesterday, I did notice that I hardly backed off the mixture screws at all maybe 1/2 turn.

What are your thoughts?

 

[stancalame]

Sorry guys I was using voice recognition on my phone preview the post The Holly re-new kid I put in two years ago came with some blue gaskets but I can't remember if it was on fuel bowl.

I assume I can tell with power valve blow by taking it out?

 

[alldodge]

With 15 in Hg and bouncy tells me you have late ignition timing.

 

[stancalame]

I thought I'd nailed the timing. I can post a couple videos later. I'm at 8 to 9? before top dead center If I am reading the timing grid correctly On the Chevy 350 by Timing mark is on 8? above zero.

 

[alldodge]

You should have something like this

 

[stancalame]

Yep, that's it. Timing left at 8 degrees BTDC. Should I be concerned with ~15 inches of vacuum? I believe timing is correct.

 

[Scott Danforth]

Sorry guys I was using voice recognition on my phone preview the post The Holly re-new kid I put in two years ago came with some blue gaskets but I can't remember if it was on fuel bowl.

I assume I can tell with power valve blow by taking it out?

you can vacuum test them, (here is a link http://www.superchevy.com/how-to/74058-carburetor-valve-problems/ ) however if you turn the idle mixture screws all the way in and the motor is still running, most likely its blown

the power valve is used to enrichen the carb between idle and about mid throttle by dumping extra fuel via the power valve channel restrictor. it would be equavalent to running primary jets 6 numbers up. since they are cheap (under $15) I have a backup in my holley box (which has springs, jets, extra gaskets, etc. and usually never need to replace it (because I have a spare)

 

[Scott Danforth]

Yep, that's it. Timing left at 8 degrees BTDC. Should I be concerned with ~15 inches of vacuum? I believe timing is correct.

I would be concerned about the non-steady vacuum which would be either a vacuum leak (manifold gasket) or a cylinder low on compression.

a steady low vacuum could be indicative of a worn motor. what were your compression numbers?

 

[alldodge]

Yep, that's it. Timing left at 8 degrees BTDC. Should I be concerned with ~15 inches of vacuum? I believe timing is correct.

Could be the motor is tired, might check compression

 

[stancalame]

I'll have to check compression. Motor is a a crate motor put in a couple years before I bought boat. I dont know history.

Any links on what I should expect for compriession? My neighbor is longterm mechanic and likely has a compression tester.