Carb jetts and bad dyno
 

Ok I got my motors back from the dyno shop 97 squared in the carbs . But wouldnt idle.So the question is if I lower the primary to maintain idle and increase the secondary side I shouldnt have a lean condition .At 87 squared it idles like a champ. Also can I pull power off the starter selenoid in front of the motor for my electric fuel pump ?

Re: Carb jetts and bad dyno
 

The pump should be powered from the battery through the appropriate fuse or breaker along with a relay and oil pressure switch.

Re: Carb jetts and bad dyno
 

if the motor was jetted on the dyno, then leave the mains alone. They should be good (or at least close, since your boat exhaust will change it a touch from the dyno headers).

If you cannot get a good idle by playing with the mix screws, then check and make sure your power valve is "low" enough. They may be staying open.

Re: Carb jetts and bad dyno
 

Yeah like he said. Main jet should have no effect on idle. Idle screws effect idle and bad or incorrect power valve will effect idle :cool:

Re: Carb jetts and bad dyno
 

Jetting will have absolutly no effect on idle. Also check fuel bowl levels. If ones to high, it will load up at idle.

Re: Carb jetts and bad dyno
 

I know ive been harping on these carbs for a while , A couple locals have the same and or bigger set ups and run 10 jetts smaller. 97 squared seems so exteme for a 540 and a dominator . The stock jetts are 87 squared and thats what they run. And they run over 600 cubic inch motors. Although what you say makes sense. Sounds like I might try a 4.5 power valve. The boat ran fine from 1200 rpms on but the idle screws were only one turn out. I will check the floats again . I checked idle screws last night.

Re: Carb jetts and bad dyno
 

Check ignition timing. Modified engines with larger than stock cams need more initial timing. If you have a MSD set up, you limit the total advance with a bushing so you can use more initial. If you have Merc ignition, use the V6 module. It provides less total timing so you can use more initial.

Re: Carb jetts and bad dyno
 

I have the v6 module . The boat ran great but in gear at idle its crap and in neutral it was crap. It only idled at abt 1200. But with 87 jetts it idled great but that was a 10 jett size change.

Re: Carb jetts and bad dyno
 

do the mix screws have any effect when cruising or is that strictly main jetts and air intake

Re: Carb jetts and bad dyno
 

Does it idle ok for a minute & then drop off ? If so it could be water reversion. the difference being on the dyno or in the boat.

Re: Carb jetts and bad dyno
 

Idle screws only affect idle mixture. As soon as the carb butterflies start to open, they're done. The power valve could be open or it could be torn. You want to use a low number p.v. on a Dominator. I was just about to change to a 4.5 when the season ended. Float bowl level is important too. A leaking needle and seat will cause the level to get too high also.

Re: Carb jetts and bad dyno
 

Thanks for the info.

Re: Carb jetts and bad dyno
 

I just reread some of your old posts about this engine. It is just like my old one except I had Merlin heads. The cam is very large and it is difficult to get to idle. I used a 1" thick 4 hole spacer to improve signal to the carb. Initial ignition timing was set at 17 degrees. MSD dist provided another 19 for a total of 36. The cam was installed with 4 degrees of advance. Compression ratio was about 9.5. If you have less it will hurt. This cam does like a lot of compression.

Re: Carb jetts and bad dyno
 

Set your SECONDARY butterflies so that they are open enough to slide a tag wire through the opening when they are resting on their stop adjustment screw.

Get a vaccuum gauge and adjust the air idle mixture screws one at a time at each corner until you attain max vaccuum. Then keep repeating the process until no more vaccuum can be attained.

Your power valve on the primary side should be approx 2 sizes under the max vaccuum reading at idle to come in supply the correct amount of fuel when on the primary circuit.

If the engine has trouble idling after setting the idle mixture, slowly adjust the SECONDARY butteflies to open them. This will increase the air to the engine while NOT providing more fuel and will increase your idle RPM...

I have an iron headed 540 with the same cam (0.632" lift)

Re: Carb jetts and bad dyno
 

OK, a little checklist:

1.Fuel bowl levels is ok.

2.Use a vaccuum gauge and adjust the air idle mixture.

3.Check if the power valve is ok, a bad PV can be hard to see so just put in new ones.

4.Check your timing, when do the "weight" go in ?

If all above is OK then there is an old dragracing trick, drill a small hole in the primary butterflies to increase the vaccum trough the carb at idle.

Do NOT mix with the jets after the dyno, they are fine.

/Orkyl

Re: Carb jetts and bad dyno
 

Yea, I tried the drill hole trick in my last blower motor . Worked quite well actually.

Re: Carb jetts and bad dyno
 

It migh work, a cheap test.