'78 470 hard starting / backfiring through intake

[RammaR]

Hi -- I've got my dad's old '78 Ranger fish and ski that I've been restoring and it's back on the water now. It runs great with a few RPM's but it won't stay idling and is hard to start. Points and condenser are fairly new and recently cleaned and plug wires are correct. I'm thinking it's a fuel supply issue.

The boat's 35 years old and I don't know that much has been done to the carbs, but it has sat for most of the last 5 years. Was only out a few times over that span. Part of the rebuilt involved pulling the motor and fuel tank to repair rotted wood - so it's not a bad gas issue.

I'm looking at a carb rebuilt kit like this one. But the fuel pump is also that old and may be weak. It doesn't look like there are rebuild kit options for the diaphragm / pump itself. New ones are a little pricey but a few friends are suggesting converting it over to an electric pump and safety switch on the oil pressure. Something like this or one from Airtex.

Has anyone rebuilt their old mechanical pump or just like the Pertronix ignition, should it meet the digital / electronic age?

I'm gonna pull the flame arrester and try to look down the carb for 2 steady fuel streams to help diagnose the issue, but looking into my options in advance.

Thanks.

 

[Another retired guy]

Re: '78 470 hard starting / backfiring through intake

Possible causes for backfiring through carb -
Timing off
Bad intake valve(s)
Ignition (intermittent loss of spark)

 

[bonzoscott]

Re: '78 470 hard starting / backfiring through intake

As Retired Guy said - if the timing is on, bad valves may be the case. Don't go searching more with out a compression test. Description and age indicates valves. Compression test would be the first thing.

 

[RammaR]

Re: '78 470 hard starting / backfiring through intake

Thanks, I'll give that a shot. Really hoping it's not a valve though.

 

[stonyloam]

Re: '78 470 hard starting / backfiring through intake

That may be the right carburetor kit. There are two different versions of the Mercarb the major difference being the accelerator pump. Study the photos #12 here: http://www.mercruiserparts.com/Show...r=50&bdesc=CARBURETOR(MERCARB)+(165-170-3.7L) (just click on the hypertext) one has a straight shaft, the other has a shaft with a 90 degree bend at the top. Both kits are available, just make sure you get the right one. Your problem could be with the idle mixture screw in the carb base (Mercarb has 1 Rochester has 2). The initial setting is to turn the screw all thee way in until it just seats, then back it out 1 1/4 turns, and make adjustments from there. Good luck.

PS Probably should do a compression check and cooling system pressure check to determine the overall engine health. Also change that impeller in the outdrive cooling is critical on that engine.

 

[stonyloam]

Re: '78 470 hard starting / backfiring through intake

If this is your fuel pump there appears to be a diaphragm kit available. http://www.mercruiserparts.com/Show...URETOR+ASSEMBLY,+FLAME+ARRESTOR+AND+FUEL+PUMP

 

[NHGuy]

Re: '78 470 hard starting / backfiring through intake

Stony is on a good thought. Since it won't idle and needs some r's to keep running you probably are not getting fuel at idle. If you seat and then reset the idle mixture screws there is a possibility it will run. Gotta try.
The backfiring can be from valves or ignition. You can check the wires for arcing by getting the motor running in a shaded area then spray a fine mist of water on the wires and watch them to see if any sparks jump out of the wires to ground.
Or if the wires are old replace them anyway. It needs doing every so often.

 

[RammaR]

Re: '78 470 hard starting / backfiring through intake

Rebuilt the water pump in the lower unit late last fall when I decided it was worth doing the gutting and rebuilding work. When we had the engine out we replaced the main pump gasket and oil pan gasket. Impeller looked brand new and nothing bad in the oil pan, I was actually pleased with how clean it was. I will check on the exact carb model tomorrow, thanks for the tips on the two models.

Thanks for the link on the fuel pump, hadn't seen that one before. My pump has the filter on top instead of down like that. I'll look into that part number though and see if it will work!

 

[RammaR]

Re: '78 470 hard starting / backfiring through intake

Here's a quick update. I definitely have the Rochester carb (upper case says "2 JET Rochester" and there are 2 idle mix screws at the base). There is a little orange tag with the part number 1376-5990A1, from what I can tell that means the carb rebuild kit link I posted originally for the Sierra 18-7076 is correct. Any doubts? It looks like that note pertains to two different styles of Mercarb, but not the Rochester - right?

Also, I noticed that the large air butterfly doesn't open regardless of throttle position. Observed with the engine off. I'm sure it's a newbie question but is that normal? I can post a video if needed, but I see the throttle linkage move but the butterfly doesn't do more than wiggle.

Thanks.

 

[Another retired guy]

Re: '78 470 hard starting / backfiring through intake

Here's a quick update. I definitely have the Rochester carb (upper case says "2 JET Rochester" and there are 2 idle mix screws at the base). There is a little orange tag with the part number 1376-5990A1, from what I can tell that means the carb rebuild kit link I posted originally for the Sierra 18-7076 is correct. Any doubts? It looks like that note pertains to two different styles of Mercarb, but not the Rochester - right?

Also, I noticed that the large air butterfly doesn't open regardless of throttle position. Observed with the engine off. I'm sure it's a newbie question but is that normal? I can post a video if needed, but I see the throttle linkage move but the butterfly doesn't do more than wiggle.

Thanks.

The large butterfly plate is the choke plate and it should be closed when engine is cold. It opens as the engine warms up.

 

[stonyloam]

Re: '78 470 hard starting / backfiring through intake

There is a little orange tag with the part number 1376-5990A1, from what I can tell that means the carb rebuild kit link I posted originally for the Sierra 18-7076 is correct.

Looks like it!

 

[RammaR]

Re: '78 470 hard starting / backfiring through intake

Been out of town for a bit, but finally got back to working on the boat today. Here's what I found:

After running the engine for a few minutes, pulled the flame arrestor and had a helper move the throttle. There were two short duration but solid streams of fuel into the throat of the carb. So that seems like the accelerator pump is functioning normally. Did it a few times and the stream pattern was consistent for both.

Compression check:
#1 - 155 psi
#2 - 159 psi
#3 - 150 psi
#4 - 153 psi

So that looked very consistent to me. Looking at the spark plugs (NGK R BPR6FS) we noticed that #1 looked a little different than the others.

Cyl #1 - white center and a little ash or build-up on the curved arm.

Cyls #2, 3, 4 - what I would consider more normal in appearance. Darker and a little more carbonaceous looking.

The gaps were slightly off too. #1 was right at 0.035, #2 was a little smaller around 0.032, #3 was right at 0.035, and #4 was most open at almost 0.040. We took some mild sandpaper and cleaned up #1 to remove the build-up and lightly cleaned up the others and put them back in.

After doing that and making sure all of the wires were connected well, it started and idled noticeably better but not perfect yet. So I do think the gap or the condition of the plugs was indicative of a timing / ignition issue.

Next up was a vacuum test. I connected the gauge I have to the port on the intake manifold just aft of the carb. I was expecting somewhere around 16-22 per the gauge instructions and Seloc manual. What we found was the needle continuously jumped / swung from about 6 to 15 on the gauge. It's hard to explain, so here is a link to a video of what it was doing.

Thoughts? I'm leaning towards timing and ignition. I don't have a dwell meter (yet). For a jumping meter like that, it says it could be a sticky valve, but wouldn't that show up on the compression test? All of those numbers look good and consistent.

Oh, meant to also say that it is idling (on muffs, out of gear) at just over 500 rpm on the tach. Increasing rpm didn't seem to make much difference on the vacuum gauge. Loosening up the fitting to create a leak did drop the need lower.

I know it's a lot of information to take in -- but thanks for all the help and advice so far.

 

[bonzoscott]

Re: '78 470 hard starting / backfiring through intake

Was it up to temp when vacuum checked? Those numbers may be for a bit higher RPM. Compression sounds good so I'm not sure the vacuum test is crucial.

 

[RammaR]

Re: '78 470 hard starting / backfiring through intake

The engine had run a little but it wasn't up to temperature. The gauge was still low on the range. I didn't think temp was important to a good vacuum reading, so maybe that contributed.

I increased the thottle some and the overall vacuum reading did go a little higher but the needle didn't settle down it jumped through a similar range.