Briggs camshaft replacement

[Mr Mower Man]

I have an interesting project in my shop right now. I have a Briggs engine (121607-0218-E1, date code 981216FA) that was starting hard due to an ACR failure on the camshaft, so I need a new cam (p/n 694035). Problem is, it's NLA, and I'm having extreme difficulty in finding one. I noticed that on 03/08/99, the camshaft used was changed to p/n 694039. I started to wonder if this revised camshaft might work in the engine in my shop, which was made before that date. So I scrolled through the entire parts list and discovered that one other part, the main jet in the carburetor, was revised on the same day (692677 changed to 498978). This kinda makes sense, since I see from the spec drawings of the two different cams that the amount of valve lift and the exact angles at which it occurs is different. So I can see why a tweak in the fuel mixture might be called for.

So here's my thought: if Briggs & Stratton changed the camshaft and main jet at the same time on the production line on 3/8/99, then I can do the same thing today and be just fine.

I also noticed that the main jet underwent another revision in 2003 (498978 changed to 498981), but I couldn't find another part in the whole engine that was changed at the same time. So it looks like I have two different main jets to choose from, and my inclination is to go with the '99 revision instead of the '03 revision.

I can't think of a reason this modification won't work, but what do you think?

[KE4AVB]

Sounds like you got access the spec drawings that I don't have. From your info it does sound possible if these were the only changes.

There will be a power curve difference base on cam angle changes. I did a little work changing cams out in '78 Malibu which did made a world difference in its performance overall but I have not even had the resources for such experiments in the last 10 years on anything else.

It would interesting to know the results.

[Mr Mower Man]

KE4AVB, Briggs & Stratton sent the spec drawings to the tech support guy at my distributor, who sent it on to me. Most of it went over my head, but I could tell there were some differences, like the amount of lift and the angles at which it occurs. Good point that the power curve will probably change. I hadn't thought of that. However, I doubt that will make any practical difference on this DR Trimmer. I just don't see the owner noticing a slight power reduction, if that's what indeed happens. But it could mean an increase in power, which would also likely go unnoticed.

I was initially undecided as to what main jet I would use, the one Briggs & Stratton went to in '99 or the one they started using in '03. After thinking about it for a little bit, I think I'll go with the main jet that was used in production starting in '99. I don't know why it was revised again in '03, but I thought that it just might be due to some other parts that were changed from '99 to '03. Just a wild guess, really don't have a clue. But since I'm not changing anything else on the engine, I'll stick with the original revision.

So I plan to order my new cam and main jet today. Unlike a lot of people on this forum, I will indeed followup on this and let you know the outcome.

[Mr Mower Man]

OK, true to my word, I'm letting you know the outcome of my experiment. I put in the revised cam (694039 instead of the original 694035) and main jet (498978 instead of the original 692677) yesterday, and the engine runs terrifically! I really had no reason to think it wouldn't work. I simply did the same thing in my shop that Briggs & Stratton did on the production line, changing those two parts simultaneously.

My only question is how it would run with the original main jet. I only briefly looked at them side by side (I couldn't tell a difference), and I didn't try running the engine first with the original.

[okie]

Quick question. What did you find wrong with the compression release on your cam shaft?

Okie

[KE4AVB]

The jet should have a number stamp on it. This number is the size in millimeters in hundredths of a millimeter. Example: a jet marked 66 is 0.66mm. With info you can compare the two jets and know if the new jet is smaller (leaner) or larger (richer). Knowing the EPA requirements it is probably smaller due stricter regs.

[HondaG100]

I just wanted to say I found this thread very interesting. Congrats Mowerman on a great job of investigative repair work.

[Mr Mower Man]

Quick question. What did you find wrong with the compression release on your cam shaft?

Okie

The little spring that keeps the ACR next to the cam lobe was broken, so it was always "out of the way" in the run position. Otherwise, it was completely intact. The owner couldn't start the thing without throwing her back out! But now that the ACR is repaired, I'll be able to get the electric start function back in shape like it was originally. I seriously doubt that the little battery would have been able to overcome all that compression.

I just wanted to say I found this thread very interesting. Congrats Mowerman on a great job of investigative repair work.

I appreciate that. I learned something with this project for sure. Normally, I wouldn't do an involved repair like this on such a small engine, since it would be very expensive and the cost to replace them is generally pretty low. But I'm doing this for a friend who's received some hard knocks lately. The labor's on me, so it actually made financial sense to repair it rather than replace it.

[KE4AVB]

But I'm doing this for a friend who's received some hard knocks lately.

You will be reward down the road for doing it. I have been there myself and really appreciated those that have help me out and I return the favor whenever they need the help or pass it along.

I have been doing a senior discount program here for several years. It actual has paid off in having happier customers and they do a lot referring work to me because I have help them out.

On that ACR the problem would have been was it stayed disengaged all the time causing starting problems due to high compression and those small electric starters would have problems starting just like the larger engines do when that ACR fails or in the case OHVs valve clearances are too great defeating the ACR function.