1993 OMC 5.0 Idles at 1000rpm

[dadtodc]

Good evening folks. I have a question about my OMC 5.0. Motor is a Ford 302 and I just rebuilt it .30 over. My question is about the idle rpm and timing. I cant get below 1000 rpms on idle. The only way is if I retard the timing and adjust the idle screws low. It will idle at about 800rpms if I do this however if I shift back to neutral from forward or reverse the engine will cut off. Does boring the block .30 over make that big of a difference? The only adjustments on the Holly carb are the idle screws. Thanks!

 

[alldodge]

Does boring the block .30 over make that big of a difference?

No
It the motor was put back together correctly and used the correct timing, plugs and gap, there should be next to no issue. My guess is you either have a vacuum leak or your carb needs work. I'm leaning toward a vacuum leak first.

Don't use starting fluid, keep it off the boat. Spray WD40, break clean, even water will do the job and change how the engine runs when the spot is found. If its under the intake this can be more of an issue to find. If you can find a vacuum port on the motor or base of carb connect a gauge to it and see what you find.

If its not a vacuum leak check you carb and make sure the throttle plate on the carb is closing down when the idle adjustment screw is backed off

 

[HT32BSX115]

Good evening folks. I have a question about my OMC 5.0. Motor is a Ford 302 and I just rebuilt it .30 over.

Right. engine mechanical rebuilds don't change idle characteristics.

It might help though if you post your model number.

If your boat is a 93, you could have one of the models below.

502APLJVB 1993
502APRJVB 1993
502BPRJVB 1993
50FAPLJVB 1993
50FAPLJVN 1993
50FAPRJVB 1993
50FAPRJVN 1993

502APLAMH 1992
502APLAMK 1992
502APRAMH 1992
502APRAMK 1992
502BPRAMH 1992
502BPRAMK 1992

It will idle at about 800rpms if I do this however if I shift back to neutral from forward or reverse the engine will cut off.

Once you find any vacuum leaks and seal them, adjust the idle with all throttle cables disconnected before you try to get the throttle cables adjusted.

Anything much over the recommended idle RPM will eventually damage the drive.

Regards,


Rick

 

[dadtodc]

Thanks to both of you. Model number is 502aprjvb. I rebuilt the carb and the motor myself. I am not an expert, but I can handle myself around motors. I hadnt even thought about a vacuum issue GAH! So here are a couple issue I found. #1 I noticed there is a line broken off of the Decelerator valve. That needs to be plugged. What exactly does that thing do anyway? Secondly there is a small oil leak on rear of the intake right in the center. Oil is actually seeping out of it and pooling up. Very slowly and without pressure. I was very careful with the intake and even put the gasket with the RTV. Looks like there is no gasket or RTV in that one spot of the intake. just dont get that.

 

[HT32BSX115]

#1 I noticed there is a line broken off of the Decelerator valve. That needs to be plugged. What exactly does that thing do anyway? Secondly there is a small oil leak on rear of the intake right in the center. Oil is actually seeping out of it and pooling up. Very slowly and without pressure. I was very careful with the intake and even put the gasket with the RTV. Looks like there is no gasket or RTV in that one spot of the intake. just dont get that.

To be honest, I had to look that one up. I have never had a deceleration valve on any engine I have ever had. It's an emission device connected to the PCV valve. It likely maintains a vacuum on the PCV to keep crankcase gasses flowing into the carb and not outside.

I don't think it's worth removing though, just reconnect the hoses and make sure there's no leaks. vacuum leaks will increase rpm (because you have to rich'en up the idle mixture to get it to idle smoothly)

My previously installed 460 had only a PCV valve. If you disconnected it, the engine idle would increase.

Evidently lots of engines have those decel valves....... I think you could remove it altogether but mind the connections to the PCV valve and close off all the vacuum leaks.

 

[alldodge]

Not an OMC or VP guy but have never seen a deceleration valve on any marine motor. Have seen dash pots on 4 bbl carbs which use vacuum secondary's. They keep the secondary's from opening until the rpms come up

 

[dadtodc]

I had to look it up s well. putting it back together is not an option. There are several barbed ports on the Decel and the smallest one snapped off flush haha just my luck. So there is not an option to put it back. The valve connects directly to the Carb though. It is crazy. I will try a screw in the rubber connection hose with some rtv and maybe try to tap the decel valve and insert something to connect to. Since it seems to be something of a mystery I will post pics to this post so the next person kows what to do if they screw up like I did. haha

 

[HT32BSX115]

Not an OMC or VP guy but have never seen a deceleration valve on any marine motor. Have seen dash pots on 4 bbl carbs which use vacuum secondary's. They keep the secondary's from opening until the rpms come up

Yeah, it's actually in the OMC (carburetor) parts list. I really think one could just remove it though.

 

[Gilded_Splinters]

Once you eliminate the possibility of vacuum leaks in obvious places, have a look at one last thing. I don't know what Holley carb you have, but some marine models have a set screw in the base that basically cracks the secondaries open a bit to let a little air in. This screw is visible on the bottom plate of the carb on the port side, provided your engine's front faces forward.

The spec is a quarter turn clockwise once the set screw contacts it's lever that actuates the secondaries. This is often too much. This can be done with the carb in place, but it's not easy to get a tool in there. It's a slotted set screw. I replaced mine with socket head so that I could tweak this easily if needed.

 

[JKNations]

You may have already checked this. What type of distributor are you using? Does it have an electronic ignition module? If so, I have found that a module has a low and high rpm circuit. Engine idles high and as you lower to get proper rpm it completely shuts down. I ran into this on my old hot rod truck and pulled my hair out trying to figure out. Replaced module and that was it. Idles perfect now. Just FYI.

 

[HT32BSX115]

You may have already checked this. What type of distributor are you using? Does it have an electronic ignition module? If so, I have found that a module has a low and high rpm circuit. Engine idles high and as you lower to get proper rpm it completely shuts down. I ran into this on my old hot rod truck and pulled my hair out trying to figure out. Replaced module and that was it. Idles perfect now. Just FYI.

Howdy,

Welcome aboard!

Well, the ignition module for his distributor (if it's OEM) is OMC #097194. (NLA) replaced by Volvo Penta 3854077 also (NLA) .
If the OP needs to replace it, evidently Pertronix has a module that will work with that dist. (IG1589)

There is a discussion of it here


Cheers,

Rick

 

[Gcotton07]

Did anyone ever figure this out ?im having the same issue