Hunting season over for my Onan!

[wheeledhorseman 579]

The Onan 16 hp in my 316 has 1750 hours on the engine clock. I got the tractor last season as a runner though in poor cosmetic condition. No complaints - it did run and I instantly fell in love with the sound of an Onan. The one issue that reared its head that was that when it heated up working hard the engine would start to hunt. Annoying at first then it got to be so bad it was like riding a kangaroo.

 

Weak mixture I hear you say, well I checked the carb and found it clean like new inside but used liberal amounts of carb cleaner anyway. You have to dismantle a lot of stuff including taking the inlet manifold off to get the carb off and having read in a thread that the manifold can cause the problem gave it a close inspection. It consists of two diecastings sealed together. There were no visible signs but I spread some sealant along the seal line anyway. Reassembled everything, fingers crossed, seemed better but the hunting when hot didn't go away in fact it seemed to get worse. Discovered that a PO has fitted the wrong plugs i.e. ones with the sealing gasket rather than chamfered fit but that made no difference to the hunting. Job for the winter I thought and finally got round to it this week.

 

There's been many comments about the importance of valve clearances on these engines so thought I'd do the full works but stopped short of head decarbonising on this tour. There was a request for photos in one thread so I've taken a few.

 

This is not a quick job but it's not particularly difficult, just time consuming which tends to be the case with twins. To get to where you want to be means first removing the air box.

 

Two bolts + three thin ones that secure it to the carb. There's a couple of small engine tins to remove, then the muffler burn guard.

 

The muffler is secured not only by the exhaust manifold bolts but also a plate that sits behing the top pto bracket so the two bolts securing it have to be removed also.

 

With all this out of the way, disconnect the throttle linkage from the cab, choke cable and fuel line. Now finally the inlet manifold can be removed complete with carb - two bolts at each end.

 

I've started to clean things up round the rear cylinder block - the front area shows what it was like, years of dust build up.

Both ends cleaned up and both valve cover plates exposed. There are three cover plate gaskets and as there was a chance they might not be reusable I'd ordered in a new set just in case.

 

 

 

 

The rear cover plate popped off without splitting the gasket. As far as I could tell there's a good chance this had not been removed since fitted at the factory 1750 hours, or if the clearances were checked at 1000 hours then it's been another 750 hours since. The inlet was spot on, the exhaust was wide by about three thou. Easy to adjust with two wrenches.

 

The front cylinder top cover plate popped off  cleanly, again without splitting the gasket but this reveals the crank case vent valve plate. I've put the bolt and spring back without the top cover plate in the photo to show how the breather valve works. The little spring steel valve was inspected and showed no signs of damage to the critical surface.

The lower valve plate came off cleanly by getting my finger nails under it. This time the inlet was a thou too close and the exhaust valve clearance perhaps three thou too small, The plates with gaskets still attached were degreased and reassembled with some gasket sealant - I use Hylomar Blue but each to their own. Didn't need the new gaskets after all but I wasn't going to risk it.

The carb was detatched from the manifold but before opening it up I tried tapping my way round the edge to see if a change of sound might indicate where the seal was broken. It was inconclusive but if there was an issue it sounded as though it might be at each end.

The two diecast halves were factory bonded together by silicone sealant, exact locaction achieved by tiny roll pins visible in the photo above.

Couldn't find a way of pulling them out so drilling them appears to be the only solution unless you know different. Trouble is that they're hard and the slit in them causes the drill to go off course. Tried drilling from the blind side which seemed better though not perfect. The drill size was 1/8th in prep for using bind rivets for reassembly.

 

 

Actually splitting the halves wasn't easy. I used a stanley knife to begin at each port end and a spare blade to act as a wedge to keep the slit open. It was easy at each end possibly confirming that this was where the issue was. Got much harder as I worked towards the centre from each end.

The silicone separated from one side only, the reason is that there is a grooved cast into the other side giving it more surface to bond to. Removing the old silicone took an hour or so of patient work.

The next step (above) was a dry run assembly to ensure that all the rivets would go in - didn't want to discover any probs after silicone RTV was applied. It was at this stage that it became apparent that some warping had taken place. With the two halves pinched together in the middle there was a small but visible gap at each end - reason for the seal failure perhaps. Bolts were added at each end to ensure a proper seal once the silicone was applied and the halves reassembled. I also bolted the carb back onto the manifold with a new gasket and a smear of silcone sealent at this stage before leaving it to cure overnight.

Manifold back in place next day with new manifold to block gaskets again with a thin smear of silicone - a belt and braces job! If you're wondering about the zip ties in the photo, these were insurance that the warping didn't strain the silicone bond once the temporary bolts were removed before final instalation and tightening down. They were then removed. Reassembly was then the reverse of disassembly, I didn't clean the carb as I'd done this last year.

 

And........... she started so took for a run round to get it nice and hot - there wasn't a hint of hunting wher it had reached a point last year when it would start misbehaving after only a few yards so fingers crossed.

 

Hope I haven't bored you and that this is of use to someone else. Sorry about not going for the full monty i.e. decarb as well, perhaps someone else will photograph that process.

 

Andy

 

[AMC RULES 37,196]

First time I've seen the process documented with photos included...kudos for sharing.    

[dbartlett1958 99]

Andy,

 

Thanks very much for this tutorial. Pictures are priceless for something like this.

[sorekiwi 761]

Good write up and photographs.

 

When I did the manifold on my P220 I ended up tapping the lower half for (I think) 6-32 socket head cap screws.  Cant remeber what I used for a sealer, I was reluctant to use RTV because I was afraid that the fuel would break it down.

[wheeledhorseman 579]

Thanks for the 'likes' and contributions. It made me think that an epilogue was needed - not so I can have the last word, but to add in a couple things that I maybe should have included in the first place.

 

I'm almost certain that the original factory sealant/adhesive used was silicone RTV, what I omitted to say was that I used automotive grade RTV suitable for high temperature applications and stated as being suitable for inlet manifolds amongst other things. So I don't think it is likely to be affected by fuel in the long term but a lesser general purpose silicone product could well be affected by either - well spotted Mike.

 

Why do these fail? Two potential reasons were suggested by what I found.

 

A thin bead of sealant was applied at the factory by machine. The two parts were then I guess pressed together and held together by the roll pins once the press was released alowing the sealant to cure. The controlled amount of sealant ensured that only tiny amounts were extruded beyond the contact surfaces creating hardly any 'frills' of excess silicone. I discovered though that, although the grove in the casting had been filled, there are also wells round each roll pin designed for sealant and in several of these it had only partly filled them. This probably explained why my attempt to identify that I had a leak on the manifold failed. As a temporary fix I'd tried smearing a sticky sealant round the complete edge where the castings mate. I didn't think to also put some over each of the roll pins which might have worked. (not suggesting it as a repair though).

 

The other candidate as a contributory factor was the slight warping which could just have been the way the original castings were made of course but I doubt it. In theory it shouldn't happen if the manifold to block bolts were correctly tightened but the gaskets becoming compressed over time may have reduced the tension allowing the hot cold cycle over the years to distort the casting slightly.

 

I'm neither a small engine expert nor engineer but I'm glad that the pics and description of what I did and found are of use to others. I'm sure others will also develop / improve on my methods.

 

Feel free to continue chipping in.

 

 

Andy

[Anglo Traction 761]

All very sound theories and remedies Andy!

.
                                               Question- Looking at the left Cylinder position and Airflow, could the warping/leak also be contributed by Running Temp difference between the Cylinders?.
I'm a total Numpster  with Onans, but I seem to recall distant Posts about venting the Belt Guard for improving the Airflow.

[Docwheelhorse 2,740]

Excellent write-up and good job repairing it.... Now with that out of the way... I went out and gave my K341 Kohler another hug and reminded myself how good it felt when my 520-8 (yes one of those) left and I never had to look at or screw with that Da** Onan ever again!

 

Tony

[Save Old Iron 1,571]

Andy,

 

just curious, did you ever try spraying carb cleaner or presenting propane to the manifold seams to see if the running characteristics of the engine would change? The "propane enrichment" technique worked very well as a troubleshooting tool on several cars I have worked on in the past.

[Docwheelhorse 2,740]

Hmmmm.... never thought about using an unlit propane torch to find a vacumn leak.... Always went with WD40 or something similar to try and temporarily plug the leak to find it. Good Idea.... I will have to throw that in the memory banks.

 

Tony

[wheeledhorseman 579]

Chuck - I did try carb cleaner early in my investigation into possible cause but the problem is access on an Onan. You can just about hit one side of the manifold with spray but that's about it until you remove the tins and the complete muffler assembly.

 

Propane might have worked by bathing the whole area - I like the idea, but its not without risks of course. Reading up on the Onan hunting issue here on RS directed me to it being carb or manifold so having removed all the stuff including complete exhaust system and the inlet manifold to release the carb from it became a case of get on and do something about it.

 

Richard - the Onan self-destruct that's often been reported on RS was at the back of my mind whilst doing this work though I gather its a rarer occurrence with the 16hp fortunately for me. As mentioned, I'm no expert and this is my first encounter with an Onan, but here's were some observations and what went through my mind.

 

The manifold leak is in a way a design defect and I suspect started small and developed over time till the governor action becomes unstable and starts to let you know by hunting. So it would be reasonable to assume that the mixture has been weaker than it should be and the engine getting hotter than it should be for some time before you really get to notice. This could certainly be a contributory factor to any big bang theory concerning Onans.

 

The belt guard does disrupt the intended forced air flow but doesn't exactly block it any more than engine tins do. This could certainly be more of an issue with the higher hp Onans. My 16hp has done 1750 hours so far and I'm hoping now the manifold and valve clearances ares good it has plenty of life left in it yet.

 

Despite its hours, mine certainly had remained a very 'dry' engine apart from slight leakage round the oil pan. The cooling fins were amazingly clear of any obstructions despite using the tractor last season to munch through a field of foot high grass that had gone to seed. What is apparent is that the usual corrosion of the aluminium fins has taken place and I imagine that this will inevitably reduce heat transfer to the air as I doubt the loose oxide conducts anything like as well as clean aluminium. Again potentially more of an issue for the larger power Onans.

 

I only found one feature / issue that related to the rear cylinder more than the front and that was that there is a 'pocket' between the two valve guides that had completely filled with dust at the rear but much less so at the front.

 

The area which has little fins but no forced air flow identified by the red circle was full to the top with dust which had been cleaned out in the photo. The front (blue circle) very much less dust as in photo. Prob not an issue with the 16hp but again may contribute to rear cylinder failures in the larger engines.

 

I'll leave it to others to think on these things. Sorry about the length of post but I know others are looking into the big bang issue.

 

Tony - I frequently use WD40 on inlet gaskets. Only the other day helped a guy sort out probs he was having with a Willys Go Devil L-head engine. He thought I was Marvo the Magician when it took just a few minutes to diagnose the issue with WD40 which was all that was to hand in his garage.

 

 

Andy