Genny starts, runs rough and then cleans up

[lincster]

Wanted to see what others thought.

Genny just started acting like this.
Fire it up, runs rough for 5-10 min, then cleans up and runs perfect.

I have never let the carb sit for more than 1 month without running gas through it.
Change the plugs/filters etc per the schedule in the owners manual.
In fact, air filter gets changed every 150 hours with the oil and oil filter.
Changed plugs per the manual.

Any ideas?

[lincster]

clicky

I am rotating the clock spring you see on the bottom right of the carb. Where the red wires are connected.
That is what my buddy told me to rotate.

It seems to have done the trick.

I have about 6 starts on the genny, both hot and cold and it fires up.

allcool, yes, I changed out the entire choke assembly. All 1 unit. 2 large screws, take off the butterfly and unhook the vacuum hose. Easy to swap over.
I do agree this was the original problem. My old diaphragm felt different than the new one. I didn't test it under vacuum, but it felt different.

[allcool]

lincster wrote:

It will start hot, but I have to go out and rotate the choke butterfly open and then it fires right up.

What are you rotating,,? are you rotating the bi-metal choke housing..? or pushing on the choke plate itself..?

Genny chokes work a little different then a vehicle choke. Our 5500 has no fast idle, and uses a governor unlike a vehicle choke with a fast idle cam lever. So the vac breaker works a little different...


Our 5500 engine must always run at 2400rpm from start to stop, speed must remain at 2400 engine rpm, to keep the genset @3600rpm=(120v_60hz),thru its pulley/belt overdrive gearing.


So basically no 'fast/slow idle' warm-up period on our 5500. 1 rpm only on our 5500, thats 2400rpm.
The engine governor needs to advance the throttle to let its governor maintain 2400rpm within a second or 2 of start up, cold or hot. So manifold vacuum pulls open the choke valve to prevent overchoking the engine within that first second or 2 of start up.

from 5500 shop manual:
"The vacuum breaker diaphragm must not leak under a vacuum of 30 inches Hg (100 kPa). It should not take a vacuum greater than 2.4 inches Hg (8 kPa) to open the choke fully. At a vacuum of 1 inch Hg (3.2 kPa) the choke plate should be at the angle shown (Figure 8-10). If necessary, bend the link at the point shown, using two pliers."

Linc, not sure if you already changed out the vacuum breaker assembly/diaphragm..? but sure sounds like it needs a double check.
I'd look there again for a leaky rubber vac hose or ripped diaphragm, or faulty linkage..?

[baker01]

Sound like the electric choke is the issue now.
Electric choke assembly and vacuum breaker assembly (Figure
8-10). Conduct the following checks and adjustments
before replacing a carburetor/choke
assembly:
1. Apply 12 VDC across the heater terminals. The
choke should open fully in 2 to 3 minutes.
2. Check the heater alignment marks and realign
if out of alignment. The choke should be closed
at temperatures below 70° F (21° C). It may be
partly open above 70° F (21° C), but should
close almost completely when vibrated (engine
cranked).

If your electric choke is closed when hot it sounds like it is out of alignment or not getting power. try step one and see what it does. You should be able to loosen the three screws on the choke to adjust it.

[Sam_Spade]

lincster wrote:
These gennys have both. A vacuum diaphragm and an electrical circuit.

Well some testing is in order.
Do you have a multi-meter ?
Once you tweek it open, does it stay there ??

[lincster]

These gennys have both. A vacuum diaphragm and an electrical circuit.

The butterfly for the choke is the issue.
Not the butterfly for the engine.

The genny starts fine cold.

It will start hot, but I have to go out and rotate the choke butterfly open and then it fires right up.

[Sam_Spade]

lincster wrote:
Well, now it won't start hot.

The butterfly is completely closed. I don't get it. Ugh.

Which butterfly, choke or throttle ?
I assume choke.

On this engine, what actually causes the choke to open ?

On some it is air flow (engine vacuum).
On others, there is an electric element attached to the choke plate via a bi-metal "spring". When it heats up, the spring opens the choke.

You first need to determine how it is SUPPOSED to work before figuring out why it is not.

[Strabo]

lincster wrote:
Well, now it won't start hot.

The butterfly is completely closed. I don't get it. Ugh.

No idea, sry Linc. I'm thinkin circuit board now? Only because these issues are strange, I ruined a genny once because I washed the sand off of it. After $800 in repairs at a shop it was never the same, my fault.

[dedmiston]

lincster wrote:
Well, now it won't start hot.

The butterfly is completely closed. I don't get it. Ugh.

Dammit.

[lincster]

Well, now it won't start hot.

The butterfly is completely closed. I don't get it. Ugh.

[lincster]

Thanks Hap.

I have read the expected lifetime of the 5500 is 3500-4000 hours before it needs a rebuild or replaced.

Valve adjustment can be done with the unit mounted in the RV cabinet. I have never done it, but a buddy has.

Strabo, I totally agree with run it like a pony. People that brag about only have 240 hours on their 10 year old genny is nuts. That actually causes more problems than anything.
Genny's are built to be run for lots of hours.
My trailer is 11 years old and I have 1150 hours on it.

[Strabo]

Run it like a pony....I mean it. New years, 46 hours in 5 days. Prez Day, 36 hours in 4 days. Time to change synthetic oil again, keep it clean....

Over 3 hours per gallon running 91 octane with full synthetic. Over 1200 hours now. I love my genny.

[Hap_Hazard]

Lincster,

So glad to hear you got the generator running again. This site continues to amaze me with the expertise and experience of our members.

By monitoring this thread you and I both eventually repaired our troublesome generators with the final fix costing nothing. This thread caused me to do some overdue routine maintenance (I always keep oil & filters up-to-date but the plugs and fuel pump needed checking). Thanks to everyone for your feedback and insights.

A couple of additional question for those in the know out there:

1)What is the expected lifetime of the Onan 5500 generator? I know this is an estimate question.

2)Does anyone have directions for adjusting the valves on this unit? Can it be done when mounted inside the RV cabinet?

Thanks again to everyone,
Hap

[lincster]

Finally got it!!!

Replaced the entire choke assembly and now it fires immediately and purrs like a kitten.

No other adjustments needed or anything.

Easy and free!!!! With used carb from a buddy. Lol

[Strabo]

lincster wrote:
Thanks Hap.

I will just replace the choke first.
My buddy gave me 3 used carbs to try from.

I won't take the carb apart. Too many folks I have heard from out there that say they are not rebuildable.

I will report back about the choke replacement.
It will be after Prez Day weekend since I'm going to play in a couple of days!!!!!

Rgr that...