When fuel hurts a motor at our shop, it is usually one of three things: a carburetor that has stopped metering, water in the fuel, or a fuel hose that has rotted from the inside. "Stale gas" by that name shows up far less often than you would expect.
When fuel hurts a motor at our shop, it is usually one of three things: a carburetor that has stopped metering, water in the fuel, or a fuel hose that has rotted from the inside. "Stale gas" by that name shows up far less often than you would expect. The hose is the one most owners never think to check.
Quick answer
- Of 429 repair-job lines where fuel was the problem, 32% were carburetors (cleaned, rebuilt or a stuck inlet needle), 25% were water or bad fuel, 21% were a fuel pump, vapour separator tank (VST) or injector fault, and 15% were a fuel line or primer bulb that had failed.
- Water was the usual form of "bad fuel". 80 of the 106 water-or-bad-fuel lines mention water. Only a handful of notes name ethanol at all.
- 23 of the 65 fuel-line and primer-bulb lines describe a hose breaking down inside, and in several the pieces plugged a filter or the VST.
- Carburetors dominate on two-strokes and sterndrives. Four-stroke and fuel-injected outboards show more water and more pump or VST faults.
- Typical labour was about $270 for carburetor work and water or bad-fuel clean-outs, about $140 for a fuel line or primer bulb, and about $280 for a pump, VST or injector repair. That is a median, not a quote.
What we did
We searched the technician notes on every completed repair job from April 1, 2023 to October 26, 2026 for fuel-system language: stale or old fuel, varnish, water in fuel, carburetor, jets, fuel line, primer bulb, fuel filter, separator, VST, fuel pump and ethanol. That matched 812 job lines on 712 repair orders. It covers every brand we service, outboards and sterndrives, not just Mercury.
We then read every one of those notes and kept the lines where degraded, contaminated or water-laden fuel, or a fuel-system part damaged by it, was the finding. That left 435 job lines on 383 repair orders. The other 377 were left out: routine filter changes with nothing wrong, or jobs where the real fault was ignition, electrical or mechanical. Six of the 435 were unclear, so the percentages below use the 429 lines with a recorded cause.
Three limits matter:
- These are job lines, not boats. Some jobs were logged on several lines with the same note. Where the same repair order and the same note appeared on more than one line, counting that pair once gives 390 instead of 435, and the ranking does not change.
- A text match is not a diagnosis. Where a note listed several faults, we counted the one that matched the complaint or came first.
- Technicians rarely write why a carburetor was dirty. We counted a carburetor clean or rebuild as a fuel-system finding, but we cannot say how many were varnish. Only two notes use words like "jelly" or "gummed up", and only about 20 of the 429 lines mention a motor that had sat unused.
No customer data was used.
What the fuel damage actually turned out to be
| What was found |
Share of 429 lines |
Note |
| Carburetor dirty, gummed or stuck |
32% |
Cleaned or rebuilt; stuck inlet needles, plugged jets, flooding bowls |
| Water or bad fuel |
25% |
In the tank, filter, carburetor or VST; tank drained and refilled |
| Fuel pump, VST or injector fault |
21% |
Mostly pumps and VST screens on fuel-injected and four-stroke motors |
| Fuel line or primer bulb failed |
15% |
Rotted inside, cracked, split, leaking at the bulb |
| Clogged or leaking fuel filter |
3% |
Thin data |
| No fault found or operator error |
3% |
Fuel checked and fine, or a wrong install |
| Tank or supply contamination |
2% |
Sediment, rust, failed tank; thin data |
Shares are rounded, so they add to 101%.
Water beat "stale gas". Of the 106 water-or-bad-fuel lines, 80 mention water. About 23 use words like old, poor, bad or "no good" about the fuel itself. One tow-in described orange fuel after regular ethanol-blend gas sat in the boat for a week to ten days. One note on a four-stroke called the fuel "almost pure alcohol". Those are the only ethanol-flavoured findings we can point to, so we will not put a number on ethanol damage. What the records show is the result: motors that quit or would not start until the tank and lines were flushed.
Hoses fail quietly. In the fuel-line group, 23 of 65 lines say the hose was deteriorating on the inside, coming apart, or had a liner breaking up. In several, bits of hose had travelled into the high-pressure filter or the VST and plugged it. Another 15 involved the primer bulb: cracked, squished inside, or leaking.
Motor type changes the mix. The unit text does not always say carbureted or fuel-injected, so we grouped by what it does say. The four-stroke group below mixes small carbureted four-strokes with injected ones, so read it as "four-stroke and injected outboards combined". 109 of the 429 lines gave no usable motor type; they are shown as their own row so every line is accounted for (80 + 189 + 51 + 109 = 429).
| Motor type (lines) |
Carburetor |
Water or bad fuel |
Fuel line or bulb |
Pump, VST or injector |
Other |
| Two-stroke outboards (80) |
46% |
26% |
18% |
9% |
1% |
| Four-stroke and injected outboards (189) |
14% |
30% |
19% |
30% |
7% |
| Sterndrives (51) |
57% |
16% |
2% |
20% |
6% |
| Motor type not stated (109) |
41% |
19% |
13% |
15% |
12% |
Carburetors are the story on older two-strokes and on sterndrives. On the four-strokes and injected motors, water and pump or VST faults each took about 30%, and a clogged VST screen or a seized pump was a common fix.
Season. The 429 classified lines ran from April through November: 29 in April, 62 in May, 68 in June, 102 in July, 85 in August, 44 in September, 21 in October and 18 in November (the six unclear lines are not in these counts). Carburetors peak in April (14 of that month's 29). Pump and VST faults peak in July and August (48 of the 89 lines in that category), when motors are working hard. We are also busiest then, so read it as a shape, not a rate.
What it cost
Median labour before parts and HST, customer-paid jobs only (warranty lines excluded, as were lines with no labour recorded):
| Category |
Median labour |
Paid lines |
| Carburetor dirty, gummed or stuck |
$270 |
130 |
| Water or bad fuel |
$270 |
97 |
| Pump, VST or injector fault |
$280 |
82 |
| Fuel line or primer bulb failed |
$140 |
59 |
These are medians, not quotes. A job line holds everything done on that line, not only the fuel work, and parts (carburetor kits, hose, pumps, filters) are on top. We did not total parts for this review. The filter, tank and no-fault categories have fewer than ten paid lines each, too few for a figure we would stand behind.
What this means for you
- Squeeze your hoses. If the bulb feels mushy, shiny, cracked or squished, replace it. If the hose is soft, sticky or shedding black flecks into the filter, replace the whole run from tank to motor. Do not trust a hose just because the outside looks fine.
- Treat water as the main suspect. The common version is not old gas, it is water in the tank or a clogged separator. One note traced it to a broken tank vent.
- Use clean fuel. HBW sells ethanol-free 89 octane fuel. Our ethanol and octane guide covers what Mercury allows and why ethanol-free is the safer choice for a boat that sits.
- Do not skip the filter and separator. Replace them on the schedule in your manual. If your outboard has a clear sight bowl on the filter, check it for water. Enclosed sterndrive bilges use solid metal filter bowls for fire safety, so do not open or drain fuel filters in an enclosed bilge or near anything that can spark; leave that to us.
- Expect a carburetor to need attention at spring start-up. If a small kicker motor or an older two-stroke will not start in spring, a carburetor clean or rebuild is the most likely fix in our records. Our notes do not say sitting was the cause. A few older carburetor parts were noted as no longer available, so ask early.
- Fuel-injected motor that quits under load? A failing fuel pump or a plugged VST screen was the usual finding.
- Put fuel on your winterization request. Tell us about any hard starting, fuel smell or soft primer bulb. Our winterization request guide lists what to include.
When to bring it in
Bring it in if the motor stalls or bogs under load, quits and restarts after a rest, is hard to start after sitting, smells of fuel, or has a primer bulb that goes soft. Bring it in too if you see water in the filter or separator, or if a fuel-pressure or water-in-fuel alarm sounds. If you are unsure whether it is fuel, ignition or the battery, our won't-start guide walks through the order we check.
To get it on the schedule, use the service request form at https://hbwservice.ca: Complete Service Request, drop off, done.
Sources
- HBW service records review, October 2026: technician notes on 435 job lines (383 repair orders), April 2023 to October 2026, classified by the main fault fixed (no customer data used)
About the author
Jay Harris runs Harris Boat Works in Gores Landing, Ontario, a third-generation family marina on Rice Lake, Ontario since 1947 and a Mercury dealer since 1965.
